diff --git a/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/association/DataAssociationManagementView.js b/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/association/DataAssociationManagementView.js
index 356daf94a8..95f9adf814 100644
--- a/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/association/DataAssociationManagementView.js
+++ b/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/association/DataAssociationManagementView.js
@@ -70,8 +70,15 @@ define([
             this.mapping = this.getCurrentMapping();
             this.mappingSync = this.getSyncNow();
             this.data.numRepresentativeProps = this.getNumRepresentativeProps();
-            this.data.sourceProps = _.pluck(this.mapping.properties,"source").slice(0,this.data.numRepresentativeProps);
-            this.data.targetProps = _.pluck(this.mapping.properties,"target").slice(0,this.data.numRepresentativeProps);
+            // Only keep properties that actually define a "source"/"target"; a leading
+            // undefined value would break the sample search query. See discussion #186.
+            // Use native Array filter/slice (lodash 3 _.first ignores the count arg).
+            this.data.sourceProps = _.pluck(this.mapping.properties, "source")
+                .filter(function(source){ return !!source; })
+                .slice(0, this.data.numRepresentativeProps);
+            this.data.targetProps = _.pluck(this.mapping.properties, "target")
+                .filter(function(target){ return !!target; })
+                .slice(0, this.data.numRepresentativeProps);
             this.data.hideSingleRecordReconButton = mappingUtils.readOnlySituationalPolicy(this.mapping.policies);
 
             this.data.reconAvailable = false;
diff --git a/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/behaviors/SingleRecordReconciliationView.js b/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/behaviors/SingleRecordReconciliationView.js
index b0845ad316..f40fc286a4 100644
--- a/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/behaviors/SingleRecordReconciliationView.js
+++ b/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/behaviors/SingleRecordReconciliationView.js
@@ -12,6 +12,7 @@
  * information: "Portions copyright [year] [name of copyright owner]".
  *
  * Copyright 2015-2016 ForgeRock AS.
+ * Portions copyright 2026 3A Systems, LLC.
  */
 
 define([
@@ -79,7 +80,13 @@ define([
         },
 
         setupSearch: function(){
-            var autocompleteProps = _.pluck(this.data.mapping.properties,"source").slice(0,this.getNumRepresentativeProps());
+            // Only keep properties that actually define a "source"; otherwise the
+            // first representative property may be undefined and break the sample
+            // search query (empty _sortKeys= => HTTP 500). See discussion #186.
+            // Use native Array filter/slice (lodash 3 _.first ignores the count arg).
+            var autocompleteProps = _.pluck(this.data.mapping.properties, "source")
+                .filter(function(source){ return !!source; })
+                .slice(0, this.getNumRepresentativeProps());
 
             mappingUtils.setupSampleSearch($("#findSampleSource",this.$el), this.data.mapping, autocompleteProps, _.bind(function(item) {
                 conf.globalData.testSyncSource = item;
diff --git a/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/properties/AttributesGridView.js b/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/properties/AttributesGridView.js
index e611f8a899..977004a180 100644
--- a/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/properties/AttributesGridView.js
+++ b/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/properties/AttributesGridView.js
@@ -12,6 +12,7 @@
  * information: "Portions copyright [year] [name of copyright owner]".
  *
  * Copyright 2016 ForgeRock AS.
+ * Portions copyright 2026 3A Systems, LLC.
  */
 
 define([
@@ -111,7 +112,13 @@ define([
                 this.data.mapProps = mapProps;
 
                 if (this.data.usesDynamicSampleSource) {
-                    let autocompleteProps = _.pluck(this.model.mapping.properties,"source").slice(0, this.model.numRepresentativeProps);
+                    // Only keep properties that actually define a "source"; otherwise the
+                    // first representative property may be undefined and break the sample
+                    // search query (empty _sortKeys= => HTTP 500). See discussion #186.
+                    // Use native Array filter/slice (lodash 3 _.first ignores the count arg).
+                    let autocompleteProps = _.pluck(this.model.mapping.properties, "source")
+                        .filter((source) => !!source)
+                        .slice(0, this.model.numRepresentativeProps);
 
                     mappingUtils.setupSampleSearch($("#findSampleSource", this.$el), this.model.mapping, autocompleteProps, (item)  => {
                         item.IDMSampleMappingName = this.model.mapping.name;
diff --git a/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/util/MappingUtils.js b/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/util/MappingUtils.js
index 55b04ea265..b9278d7e20 100644
--- a/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/util/MappingUtils.js
+++ b/openidm-ui/openidm-ui-admin/src/main/js/org/forgerock/openidm/ui/admin/mapping/util/MappingUtils.js
@@ -65,11 +65,14 @@ define([
 
     obj.setupSampleSearch = function(el, mapping, autocompleteProps, selectSuccessCallback){
         var searchList,
-            selectedItem;
+            selectedItem,
+            // Guard against empty/undefined entries (e.g. properties without a "source").
+            cleanProps = _.compact(autocompleteProps),
+            primaryProp = cleanProps[0];
 
         el.selectize({
-            valueField: autocompleteProps[0],
-            searchField: autocompleteProps,
+            valueField: primaryProp,
+            searchField: cleanProps,
             maxOptions: 10,
             create: false,
             onChange: function() {
@@ -96,18 +99,20 @@ define([
                 item: function(item, escape) {
                     selectedItem = item;
 
-                    return "<div>" +escape(item[autocompleteProps[0]]) +"</div>";
+                    return "<div>" +escape(item[primaryProp]) +"</div>";
                 }
             },
             load: function(query, callback) {
-                if (!query.length || query.length < 2 || !autocompleteProps.length) {
+                if (!query.length || query.length < 2 || !cleanProps.length) {
                     return callback();
                 }
 
-                searchDelegate.searchResults(mapping.source, autocompleteProps, query).then(function(response) {
-                    if(response) {
+                searchDelegate.searchResults(mapping.source, cleanProps, query).then(function(response) {
+                    if(response && response.length) {
                         searchList = response;
-                        callback([response]);
+                        // searchResults already returns an array of records; pass it
+                        // through as-is (selectize expects a flat array of options).
+                        callback(response);
                     } else {
                         searchList = [];
 
diff --git a/openidm-ui/openidm-ui-admin/src/test/qunit/org/forgerock/openidm/ui/admin/mapping/util/MappingUtilsTest.js b/openidm-ui/openidm-ui-admin/src/test/qunit/org/forgerock/openidm/ui/admin/mapping/util/MappingUtilsTest.js
index dceb72f5b0..482f438378 100644
--- a/openidm-ui/openidm-ui-admin/src/test/qunit/org/forgerock/openidm/ui/admin/mapping/util/MappingUtilsTest.js
+++ b/openidm-ui/openidm-ui-admin/src/test/qunit/org/forgerock/openidm/ui/admin/mapping/util/MappingUtilsTest.js
@@ -1,5 +1,63 @@
 define([
-    "org/forgerock/openidm/ui/admin/mapping/util/MappingUtils"
-], function (MappingUtils) {
+    "jquery",
+    "sinon",
+    "org/forgerock/openidm/ui/admin/mapping/util/MappingUtils",
+    "org/forgerock/openidm/ui/common/delegates/SearchDelegate"
+], function ($, sinon, MappingUtils, SearchDelegate) {
     QUnit.module('MappingUtils Tests');
+
+    QUnit.test("setupSampleSearch passes a flat array of records to the selectize load callback", function (assert) {
+        var done = assert.async(),
+            records = [{ email: "jsanchez@example.com", lastName: "Sanchez", firstName: "Jane" }],
+            capturedConfig,
+            selectizeStub = sinon.stub($.fn, "selectize", function (config) {
+                capturedConfig = config;
+                return this;
+            }),
+            searchStub = sinon.stub(SearchDelegate, "searchResults", function () {
+                return $.Deferred().resolve(records).promise();
+            });
+
+        try {
+            MappingUtils.setupSampleSearch(
+                $("<input>"),
+                { source: "system/hr/account" },
+                ["email", "lastName", "firstName"],
+                function () {}
+            );
+
+            assert.equal(capturedConfig.valueField, "email", "valueField is the first non-empty prop");
+
+            capturedConfig.load("Sanchez", function (options) {
+                // Regression test: options must be the flat array of records, not [[...]].
+                assert.deepEqual(options, records, "load callback receives a flat array of records");
+                done();
+            });
+        } finally {
+            selectizeStub.restore();
+            searchStub.restore();
+        }
+    });
+
+    QUnit.test("setupSampleSearch ignores props without a source (compacts valueField/searchField)", function (assert) {
+        var capturedConfig,
+            selectizeStub = sinon.stub($.fn, "selectize", function (config) {
+                capturedConfig = config;
+                return this;
+            });
+
+        try {
+            MappingUtils.setupSampleSearch(
+                $("<input>"),
+                { source: "managed/user" },
+                [undefined, "userName", "sn"],
+                function () {}
+            );
+
+            assert.equal(capturedConfig.valueField, "userName", "valueField falls back to first non-empty prop");
+            assert.deepEqual(capturedConfig.searchField, ["userName", "sn"], "searchField has no undefined entries");
+        } finally {
+            selectizeStub.restore();
+        }
+    });
 });
\ No newline at end of file
diff --git a/openidm-ui/openidm-ui-common/src/main/js/org/forgerock/openidm/ui/common/delegates/SearchDelegate.js b/openidm-ui/openidm-ui-common/src/main/js/org/forgerock/openidm/ui/common/delegates/SearchDelegate.js
index 7f6a59c626..a8b34987f6 100644
--- a/openidm-ui/openidm-ui-common/src/main/js/org/forgerock/openidm/ui/common/delegates/SearchDelegate.js
+++ b/openidm-ui/openidm-ui-common/src/main/js/org/forgerock/openidm/ui/common/delegates/SearchDelegate.js
@@ -23,12 +23,41 @@ define([
 
     var obj = new AbstractDelegate(constants.host + "/" + constants.context);
 
+    /**
+     * Builds the search URL for a resource query.
+     *
+     * Note: only the first non-empty property is used for "_sortKeys". When the
+     * supplied props array starts with empty/undefined values (e.g. a mapping
+     * whose first property has no "source"), an empty "_sortKeys=" must NOT be
+     * emitted, otherwise the backend (CREST/IDM) returns an HTTP 500 error.
+     *
+     * In addition, "_sortKeys" is omitted for system resources ("system/..."),
+     * because many connectors (e.g. the CSV connector) do not support
+     * server-side sorting and would fail the whole query. Sample searches are
+     * capped at a handful of results anyway, so the sort order is not required.
+     */
+    obj.buildSearchUrl = function (resource, props, searchString, comparisonOperator, additionalQuery, maxPageSize) {
+        var sortKey = _.find(props, function (p) { return !!p; }),
+            sortableResource = !/^\/?system\//.test(resource),
+            url = "/" + resource + "?";
+
+        if (sortKey && sortableResource) {
+            url += "_sortKeys=" + sortKey + "&";
+        }
+
+        // [a,b] => "a or (b)"; [a,b,c] => "a or (b or (c))"
+        url += "_pageSize=" + maxPageSize +
+               "&_queryFilter=" + obj.generateQueryFilter(props, searchString, additionalQuery, comparisonOperator);
+
+        return url;
+    };
+
     obj.searchResults = function (resource, props, searchString, comparisonOperator, additionalQuery) {
         var maxPageSize = 10;
 
         return this.serviceCall({
             "type": "GET",
-            "url":  "/" + resource + "?_sortKeys=" + props[0] + "&_pageSize=" + maxPageSize + "&_queryFilter=" + obj.generateQueryFilter(props, searchString, additionalQuery, comparisonOperator)// [a,b] => "a or (b)"; [a,b,c] => "a or (b or (c))"
+            "url":  obj.buildSearchUrl(resource, props, searchString, comparisonOperator, additionalQuery, maxPageSize)
         }).then(
             function (qry) {
                 return _.take(qry.result, maxPageSize);//we never want more than 10 results from search in case _pageSize does not work
diff --git a/openidm-ui/openidm-ui-common/src/test/qunit/tests/org/forgerock/openidm/ui/common/delegates/SearchDelegateTest.js b/openidm-ui/openidm-ui-common/src/test/qunit/tests/org/forgerock/openidm/ui/common/delegates/SearchDelegateTest.js
index ced34370c9..ec7b7b1b89 100644
--- a/openidm-ui/openidm-ui-common/src/test/qunit/tests/org/forgerock/openidm/ui/common/delegates/SearchDelegateTest.js
+++ b/openidm-ui/openidm-ui-common/src/test/qunit/tests/org/forgerock/openidm/ui/common/delegates/SearchDelegateTest.js
@@ -1,3 +1,18 @@
+/**
+ * The contents of this file are subject to the terms of the Common Development and
+ * Distribution License (the License). You may not use this file except in compliance with the
+ * License.
+ *
+ * You can obtain a copy of the License at legal/CDDLv1.0.txt. See the License for the
+ * specific language governing permission and limitations under the License.
+ *
+ * When distributing Covered Software, include this CDDL Header Notice in each file and include
+ * the License file at legal/CDDLv1.0.txt. If applicable, add the following below the CDDL
+ * Header, with the fields enclosed by brackets [] replaced by your own identifying
+ * information: "Portions copyright [year] [name of copyright owner]".
+ *
+ * Portions copyright 2026 3A Systems, LLC.
+ */
 define([
     "org/forgerock/openidm/ui/common/delegates/SearchDelegate"
 ], function (SearchDelegate) {
@@ -12,4 +27,44 @@ define([
         assert.equal(SearchDelegate.generateQueryFilter(props, search), '(userName sw "test" or (givenName sw "test" or (sn sw "test")))', "Basic Query Filter Generated");
         assert.equal(SearchDelegate.generateQueryFilter(props, search, additionalQuery, comparisonOperator), '((userName sw "test" or (givenName sw "test" or (sn sw "test"))) and (true))', "Complex Query Filter Generated");
     });
+
+    QUnit.test("buildSearchUrl uses the first property as _sortKeys", function (assert) {
+        var url = SearchDelegate.buildSearchUrl("managed/user", ["userName", "sn"], "test", null, null, 10);
+
+        assert.equal(
+            url,
+            '/managed/user?_sortKeys=userName&_pageSize=10&_queryFilter=(userName sw "test" or (sn sw "test"))',
+            "_sortKeys is set to the first property"
+        );
+    });
+
+    QUnit.test("buildSearchUrl skips empty _sortKeys when the first property is missing (discussion #186)", function (assert) {
+        // Mapping whose first property has no "source" => leading undefined value.
+        var url = SearchDelegate.buildSearchUrl("managed/user", [undefined, "sn"], "test", null, null, 10);
+
+        assert.equal(url.indexOf("_sortKeys=&"), -1, "no empty _sortKeys= is emitted");
+        assert.equal(
+            url,
+            '/managed/user?_sortKeys=sn&_pageSize=10&_queryFilter=(sn sw "test")',
+            "_sortKeys falls back to the first non-empty property"
+        );
+    });
+
+    QUnit.test("buildSearchUrl omits _sortKeys entirely when no property is usable", function (assert) {
+        var url = SearchDelegate.buildSearchUrl("managed/user", [undefined, ""], "test", null, null, 10);
+
+        assert.equal(url.indexOf("_sortKeys"), -1, "no _sortKeys parameter is present at all");
+        assert.equal(url.indexOf("?_pageSize=10"), "/managed/user".length, "the query string starts directly with _pageSize");
+    });
+
+    QUnit.test("buildSearchUrl omits _sortKeys for system resources that may not support sorting", function (assert) {
+        var url = SearchDelegate.buildSearchUrl("system/hr/account", ["email", "lastName"], "Sanchez", null, null, 10);
+
+        assert.equal(url.indexOf("_sortKeys"), -1, "no _sortKeys parameter is sent to a system connector");
+        assert.equal(
+            url,
+            '/system/hr/account?_pageSize=10&_queryFilter=(email sw "Sanchez" or (lastName sw "Sanchez"))',
+            "system resource query has no _sortKeys but keeps the query filter"
+        );
+    });
 });