diff --git a/fsw/cfe-core/src/es/cfe_es_resource.c b/fsw/cfe-core/src/es/cfe_es_resource.c
index 39a1ebaca..82ae36a77 100644
--- a/fsw/cfe-core/src/es/cfe_es_resource.c
+++ b/fsw/cfe-core/src/es/cfe_es_resource.c
@@ -49,6 +49,11 @@
  */
 int32 CFE_ES_ResourceID_ToIndex_Internal(uint32 Serial, uint32 TableSize, uint32 *Idx)
 {
+    if (Idx == NULL)
+    {
+        return CFE_ES_ERR_BUFFER;
+    }
+
     if (Serial > CFE_ES_RESOURCEID_MAX)
     {
         return CFE_ES_ERR_RESOURCEID_NOT_VALID;