--- a/js/src/jsdbgapi.cpp
+++ b/js/src/jsdbgapi.cpp
@@ -2271,17 +2271,17 @@ public:
     bool isEmpty() {
         return (mReadPos == mWritePos);
     }
 
     EthogramScriptEntry *addScript(JSContext *cx, JSObject *obj, char *filename, JSString *jsfilename) {
         JSHashNumber hash = JS_HashString(filename);
         JSHashEntry **hep = JS_HashTableRawLookup(traceVisScriptTable, hash, filename);
         if (*hep != NULL)
-            return JS_FALSE;
+            return NULL;
 
         JS_HashTableRawAdd(traceVisScriptTable, hep, hash, filename, this);
 
         EthogramScriptEntry * entry = (EthogramScriptEntry *) JS_malloc(cx, sizeof(EthogramScriptEntry));
         if (entry == NULL)
             return NULL;
 
         entry->next = mScripts;