Problem with exp() function

I'm sure this is a trivial question, but I'm relatively new to MathCad Prime.

Can anybody see whats wrong with this:

The value for m4 is computed earlier in the worksheet (obviously, as otherwise mr would be invalid.  I have not put units on k (JK-1) of E (J) but it doesn't make any difference anyway.

So why won't it compute Ppen?  Is it becase Eg and E are so small?  Or, more likely, what am I doing wrong?