diff --git a/src/main/java/org/spdx/spreadsheetstore/SpreadsheetStore.java b/src/main/java/org/spdx/spreadsheetstore/SpreadsheetStore.java
index cbc8cf4..37ef32d 100644
--- a/src/main/java/org/spdx/spreadsheetstore/SpreadsheetStore.java
+++ b/src/main/java/org/spdx/spreadsheetstore/SpreadsheetStore.java
@@ -359,7 +359,7 @@ private Map<String, String> copyPackageInfoToSS(String documentUri,
 			PackageInfoSheet packageInfoSheet, ModelCopyManager copyManager,
 			Map<String, Collection<ExternalRef>> externalRefs,
 			Map<String, Collection<Relationship>> allRelationships, Map<String, Collection<Annotation>> allAnnotations) throws InvalidSPDXAnalysisException {
-		Map<String, String> fileIdToPkgId = new HashMap<>();
+		Map<String, List<String>> fileIdToPkgIds = new HashMap<>();
 		List<SpdxPackage> packages;
 		
 		try (@SuppressWarnings("unchecked")
@@ -372,14 +372,7 @@ private Map<String, String> copyPackageInfoToSS(String documentUri,
 			String pkgId = pkg.getId();
 			Collection<SpdxFile> files = pkg.getFiles();
 			for (SpdxFile file:files) {
-				String fileId = file.getId();
-				String pkgIdsForFile = fileIdToPkgId.get(fileId);
-				if (pkgIdsForFile == null) {
-					pkgIdsForFile = pkgId;
-				} else {
-					pkgIdsForFile = pkgIdsForFile + ", " + pkgId;
-				}
-				fileIdToPkgId.put(fileId, pkgIdsForFile);
+				fileIdToPkgIds.computeIfAbsent(file.getId(), k -> new ArrayList<>()).add(pkgId);
 			}
 			Collection<ExternalRef> pkgExternalRefs = pkg.getExternalRefs();
 			if (pkgExternalRefs != null && pkgExternalRefs.size() > 0) {
@@ -395,6 +388,10 @@ private Map<String, String> copyPackageInfoToSS(String documentUri,
 				allAnnotations.put(pkg.getId(), annotations);
 			}
 		}
+		Map<String, String> fileIdToPkgId = new HashMap<>();
+		for (Entry<String, List<String>> entry : fileIdToPkgIds.entrySet()) {
+			fileIdToPkgId.put(entry.getKey(), String.join(", ", entry.getValue()));
+		}
 		return fileIdToPkgId;
 	}