diff --git a/gc.c b/gc.c
index 07b8d76efb..016b948fed 100644
--- a/gc.c
+++ b/gc.c
@@ -974,9 +974,9 @@ gc_mark_children(VALUE ptr, int lev)
 	break;
 
       default:
-	rb_bug("rb_gc_mark(): unknown data type 0x%lx (%p) %s",
+	rb_bug("rb_gc_mark(): unknown data type 0x%lx(%p) %s",
 	       obj->as.basic.flags & T_MASK, obj,
-	       is_pointer_to_heap(obj) ? "corrupted object" : "not a pointer to heap");
+	       is_pointer_to_heap(obj) ? "corrupted object" : "non object");
     }
 }