想在vntraderpro加载价差交易模块，出现以下异常提示：
Traceback (most recent call last):
File "c:\vnstudio\lib\site-packages\vnstation\cli.py", line 92, in run_trader
main_engine.add_app(app)
File "c:\vnstudio\lib\site-packages\vnpy\trader\engine.py", line 96, in add_app
engine = self.add_engine(app.engine_class)
File "c:\vnstudio\lib\site-packages\vnpy\trader\engine.py", line 71, in add_engine
engine = engine_class(self, self.event_engine)
File "c:\vnstudio\lib\site-packages\vnpy\app\spread_trading\engine.py", line 51, in init
self.strategy_engine: SpreadStrategyEngine = SpreadStrategyEngine(self)
File "c:\vnstudio\lib\site-packages\vnpy\app\spread_trading\engine.py", line 608, in init
self.load_strategy_class()
File "c:\vnstudio\lib\site-packages\vnpy\app\spread_trading\engine.py", line 630, in load_strategy_class
self.load_strategy_class_from_folder(path2, "strategies")
File "c:\vnstudio\lib\site-packages\vnpy\app\spread_trading\engine.py", line 645, in load_strategy_class_from_folder
self.load_strategy_class_from_module(strategy_module_name)
UnboundLocalError: local variable 'strategy_module_name' referenced before assignment
请教解决方法。 谢谢！