谁能帮我解决这个错误？

1.我在实现 games50 中的马里奥游戏时遇到了一个错误，在状态机中显示如下

    function StateMachine:change(stateName, enterParams)

        assert(self.states[stateName]) -- state must exist!
        self.current:exit()
        self.current = self.states[stateName]()
        self.current:enter(enterParams)
    end

错误

src/StateMachine.lua:18: attempt to call method 'enter' (a nil value)

Traceback

src/StateMachine.lua:18: in function 'change'
main.lua:21: in function 'load'
[C]: in function 'xpcall'
[C]: in function 'xpcall'