diff --git a/src/raptor_parse.c b/src/raptor_parse.c
index 26911f47..6caa7f1c 100644
--- a/src/raptor_parse.c
+++ b/src/raptor_parse.c
@@ -252,12 +252,12 @@ raptor_world_get_parser_factory(raptor_world *world, const char *name)
  *
  * Get number of parsers
  *
- * Return value: number of parsers
+ * Return value: number of parsers or <0 on failure
  **/
 int
 raptor_world_get_parsers_count(raptor_world* world)
 {
-  RAPTOR_ASSERT_OBJECT_POINTER_RETURN_VALUE(world, raptor_world, NULL);
+  RAPTOR_ASSERT_OBJECT_POINTER_RETURN_VALUE(world, raptor_world, -1);
 
   raptor_world_open(world);
 
diff --git a/src/raptor_serialize.c b/src/raptor_serialize.c
index a1f29d78..a0344418 100644
--- a/src/raptor_serialize.c
+++ b/src/raptor_serialize.c
@@ -235,12 +235,12 @@ raptor_get_serializer_factory(raptor_world* world, const char *name)
  *
  * Get number of serializers
  *
- * Return value: number of serializers
+ * Return value: number of serializers or <0 on failure
  **/
 int
 raptor_world_get_serializers_count(raptor_world* world)
 {
-  RAPTOR_ASSERT_OBJECT_POINTER_RETURN_VALUE(world, raptor_world, NULL);
+  RAPTOR_ASSERT_OBJECT_POINTER_RETURN_VALUE(world, raptor_world, -1);
 
   raptor_world_open(world);