diff --git a/gobject/gtype.c b/gobject/gtype.c
index 9e60a1e28..f766ffdb9 100644
--- a/gobject/gtype.c
+++ b/gobject/gtype.c
@@ -3972,9 +3972,11 @@ gboolean
 g_type_check_instance_is_fundamentally_a (GTypeInstance *type_instance,
                                           GType          fundamental_type)
 {
+  TypeNode *node;
   if (!type_instance || !type_instance->g_class)
     return FALSE;
-  return NODE_FUNDAMENTAL_TYPE(lookup_type_node_I (type_instance->g_class->g_type)) == fundamental_type;
+  node = lookup_type_node_I (type_instance->g_class->g_type);
+  return node && (NODE_FUNDAMENTAL_TYPE(node) == fundamental_type);
 }
 
 gboolean