Parameter must be a Guid or string representing a Guid Error. Don't know WHY??

I am a newbee to play with python, today I meet a weird problem, I don't know why, is there some basic knowledge I haven't know?

I am creating a simple stair, when the second crv is absent, I offset the 1st one, then do the some thing. I remember that I have done some other work like this and don't have ANY ERROR, but tonight, I constantly getting errors everywhere!! I don't know whether my parameter don't fit the asked type?!

here is when I use the 2nd crv, It is OK:

here is when I offset the 1st crv to be the 2nd crv, there occure the problem: