Using call_user_function with closures

I am taking over an extension from one of my coworkers, and it was written before the time of PHP 5.3. Are there any special considerations to think about when using closures with call_user_function? This is the current line: if (call_user_function(EG(function_table), NULL, &callback, &local_retval, 1, argsv TSRMLS_CC ) == FAILURE) { However, when the extension is loaded, it crashes on this line when callback is a closure. When callback is a string (a userspace function name), everything works fine. Thanks, Alec