diff --git a/lib/common/tests/strings/pcmk__str_update_test.c b/lib/common/tests/strings/pcmk__str_update_test.c
index 3824c5594f..7d1b0cf780 100644
--- a/lib/common/tests/strings/pcmk__str_update_test.c
+++ b/lib/common/tests/strings/pcmk__str_update_test.c
@@ -1,65 +1,81 @@
 /*
  * Copyright 2022 the Pacemaker project contributors
  *
  * The version control history for this file may have further details.
  *
  * This source code is licensed under the GNU Lesser General Public License
  * version 2.1 or later (LGPLv2.1+) WITHOUT ANY WARRANTY.
  */
 
 #include <crm_internal.h>
 
 #include <crm/common/unittest_internal.h>
 
+#include "mock_private.h"
+
 static void
 update_null(void **state) {
     char *str = NULL;
 
     // These just make sure they don't crash
     pcmk__str_update(NULL, NULL);
     pcmk__str_update(NULL, "value");
 
     // Update an already NULL string to NULL
     pcmk__str_update(&str, NULL);
     assert_null(str);
 
     // Update an already allocated string to NULL
     str = strdup("hello");
     pcmk__str_update(&str, NULL);
     assert_null(str);
 }
 
 static void
 update_same(void **state) {
     char *str = NULL;
     char *saved = NULL;
 
     str = strdup("hello");
     saved = str;
     pcmk__str_update(&str, "hello");
     assert_ptr_equal(saved, str); // No free and reallocation
     free(str);
 }
 
 static void
 update_different(void **state) {
     char *str = NULL;
 
     str = strdup("hello");
     pcmk__str_update(&str, "world");
     assert_string_equal(str, "world");
     free(str);
 }
 
+static void
+strdup_fails(void **state) {
+    char *str = NULL;
+
+    str = strdup("hello");
+
+    pcmk__mock_strdup = true;       // strdup() will return NULL
+    pcmk__assert_asserts(pcmk__str_update(&str, "world"));
+    pcmk__mock_strdup = false;      // Use the real strdup()
+
+    free(str);
+}
+
 int
 main(int argc, char **argv)
 {
     const struct CMUnitTest tests[] = {
         cmocka_unit_test(update_null),
         cmocka_unit_test(update_same),
         cmocka_unit_test(update_different),
+        cmocka_unit_test(strdup_fails),
     };
 
     cmocka_set_message_output(CM_OUTPUT_TAP);
     return cmocka_run_group_tests(tests, NULL, NULL);
 }