I wasn't able to reproduce the problem on my mac.
Normally when I see something like this, the cause is that a different variable name is used for something between the two runs. In symbolic computing the choice of a variable name matters, which is counterintuitive for most programmers in most languages. Another common cause is that a method isn't specified for a numerical function. I didn't see that though in your notebook, but please check that both of these aren't being done.