The g_return*_if_fail() calls should use PREFIX_IS_TYPE_NAME [1], which returns whether the given instance is of the right type, not PREFIX_TYPE_NAME [2], which checks whether the given instance if of the right type, warns if not, and returns the passed-in instance unchanged. Using the wrong one means the returned value is the passed-in pointer, and thus passes the g_return*_if_fail() whenever the pointer is non-NULL, no matter whether it's of the right type or not. A warning would still be emitted by g_type_check_instance_cast(), but the execution wouldn't get short-circuited. [1] calls g_type_check_instance_is_a() [2] calls g_type_check_instance_cast()