diff --git a/gobject/gtype.c b/gobject/gtype.c
index 77feb7d4a..8665db294 100644
--- a/gobject/gtype.c
+++ b/gobject/gtype.c
@@ -4161,7 +4161,7 @@ g_type_check_instance_is_a (GTypeInstance *type_instance,
     return FALSE;
 
   iface = lookup_type_node_I (iface_type);
-  if (iface->is_final)
+  if (iface && iface->is_final)
     return type_instance->g_class->g_type == iface_type;
 
   node = lookup_type_node_I (type_instance->g_class->g_type);
diff --git a/gobject/tests/type-flags.c b/gobject/tests/type-flags.c
index 23313c68e..653cb9b01 100644
--- a/gobject/tests/type-flags.c
+++ b/gobject/tests/type-flags.c
@@ -130,6 +130,8 @@ test_type_flags_final_instance_check (void)
                                               TEST_TYPE_DEPRECATED));
   g_assert_true (g_type_check_instance_is_a ((GTypeInstance *) final,
                                               G_TYPE_OBJECT));
+  g_assert_false (g_type_check_instance_is_a ((GTypeInstance *) final,
+                                              G_TYPE_INVALID));
 
   g_clear_object (&final);
 }
@@ -184,6 +186,8 @@ test_type_flags_deprecated (void)
                                               G_TYPE_OBJECT));
   g_assert_false (g_type_check_instance_is_a ((GTypeInstance *) deprecated_object,
                                               TEST_TYPE_FINAL));
+  g_assert_false (g_type_check_instance_is_a ((GTypeInstance *) deprecated_object,
+                                              G_TYPE_INVALID));
 
   g_test_assert_expected_messages ();