Call to is_subclass_of() expects a string

Using the class name directly throws an exception:

ErrorException [ 8 ]: Use of undefined constant Minion_Task - assumed 'Minion_Task'

Verified by looking at PHP source directly which confirms this should a string:

ZEND_PARSE_PARAMETERS_START(2, 3)
		Z_PARAM_ZVAL(obj)
		Z_PARAM_STR(class_name)
		Z_PARAM_OPTIONAL
		Z_PARAM_BOOL(allow_string)
ZEND_PARSE_PARAMETERS_END();


Notice only the first argument is a zval, $class_name is a string.