diff --git a/mysql-test/main/mdev40385.result b/mysql-test/main/mdev40385.result
new file mode 100644
index 0000000000000..78f7b3b2239ea
--- /dev/null
+++ b/mysql-test/main/mdev40385.result
@@ -0,0 +1,35 @@
+#
+# MDEV-40385 use-of-uninitialized-value in Binary_string::c_ptr()
+#
+# KDF() with a 4th (kdf_name) argument that is an integer literal made
+# Item_func_kdf::val_str() evaluate args[3]->val_str() into the shared
+# result buffer and then call c_ptr() on it. The integer-to-string
+# conversion does not NUL-terminate, and the buffer is not "alloced",
+# so c_ptr() (sql/sql_string.h) read an uninitialized byte.
+# Under MSAN this was flagged as use-of-uninitialized-value.
+#
+# The 4th argument that is not a valid kdf name must be rejected with a
+# warning and NULL, without reading uninitialized memory.
+#
+SELECT KDF('','',1000,256);
+KDF('','',1000,256)
+NULL
+Warnings:
+Warning	3047	Invalid argument error: '256' in function kdf.
+SELECT KDF('secret','salt',1000,999);
+KDF('secret','salt',1000,999)
+NULL
+Warnings:
+Warning	3047	Invalid argument error: '999' in function kdf.
+#
+# Valid kdf_name arguments must still work.
+#
+SELECT KDF('secret','salt',1000,'pbkdf2_hmac') IS NOT NULL;
+KDF('secret','salt',1000,'pbkdf2_hmac') IS NOT NULL
+1
+SELECT KDF('secret','salt') IS NOT NULL;
+KDF('secret','salt') IS NOT NULL
+1
+#
+# End of 11.4 tests
+#
diff --git a/mysql-test/main/mdev40385.test b/mysql-test/main/mdev40385.test
new file mode 100644
index 0000000000000..97830d54a340d
--- /dev/null
+++ b/mysql-test/main/mdev40385.test
@@ -0,0 +1,28 @@
+# MSAN only: This test verifies the fix behaviorally (warning + NULL). The uninitialized
+# read itself only triggers a failure when run on an MSAN build; on a normal
+# build the test passes on both the unfixed and fixed server.
+--echo #
+--echo # MDEV-40385 use-of-uninitialized-value in Binary_string::c_ptr()
+--echo #
+--echo # KDF() with a 4th (kdf_name) argument that is an integer literal made
+--echo # Item_func_kdf::val_str() evaluate args[3]->val_str() into the shared
+--echo # result buffer and then call c_ptr() on it. The integer-to-string
+--echo # conversion does not NUL-terminate, and the buffer is not "alloced",
+--echo # so c_ptr() (sql/sql_string.h) read an uninitialized byte.
+--echo # Under MSAN this was flagged as use-of-uninitialized-value.
+--echo #
+--echo # The 4th argument that is not a valid kdf name must be rejected with a
+--echo # warning and NULL, without reading uninitialized memory.
+--echo #
+SELECT KDF('','',1000,256);
+SELECT KDF('secret','salt',1000,999);
+
+--echo #
+--echo # Valid kdf_name arguments must still work.
+--echo #
+SELECT KDF('secret','salt',1000,'pbkdf2_hmac') IS NOT NULL;
+SELECT KDF('secret','salt') IS NOT NULL;
+
+--echo #
+--echo # End of 11.4 tests
+--echo #
diff --git a/sql/item_strfunc.cc b/sql/item_strfunc.cc
index c98dcdfeb03f1..27d4caabe5c5c 100644
--- a/sql/item_strfunc.cc
+++ b/sql/item_strfunc.cc
@@ -489,9 +489,9 @@ String *Item_func_kdf::val_str(String *buf)
   {
     if (String *s= args[3]->val_str(buf))
     {
-      if (strcasecmp(s->c_ptr(), "hkdf") == 0)
+      if (strcasecmp(s->c_ptr_safe(), "hkdf") == 0)
         use_hkdf= true;
-      else if (strcasecmp(s->c_ptr(), "pbkdf2_hmac") != 0)
+      else if (strcasecmp(s->c_ptr_safe(), "pbkdf2_hmac") != 0)
       {
         invalid_argument_error(func_name(), ErrConvStringQ(s).ptr());
         goto ret_null;