Derivative of the Real Part of a Complex Function (Mathematica)

[Swamp Thing]

TL;DR

Why doesn't this work:
D [ Re[ Exp[u + 10*I] ], u ] /. u->0.5

When I type in this:

D [
      Re[
           Exp[u + 10*I]
        ],
           u
   ]    /.  u->0.5

I get this output:

Of course, I could just put the Re outside and the D inside, but it would be nice to know what is wrong with the above. What's with the Re' in the output?

 

[jedishrfu]

Context? Language? Mathematica?

 

[Swamp Thing]

Mathematica.

 

[Dale]

Mathematica doesn’t know that you intend u to be real.