Mocking object with methods named the same as testify's ones

Hi! I need to mock an interface which has a method called On. 

```golang
type MyMockedObj struct {}

func (obj *MyMockedObj) On() error {
    arguments := obj.Called()

    return arguments.Error(0)
}


func TestThis() {
    obj := Obj{}

    obj.On("On").Return(nil) // Error: obj.On("On").Return undefined (type error has no field or method Return)
}
```

How I supposed to do that?