What is wrong with this code? (eval function, symbolic toolbox, 3 super short lines)

This code worked in Matlab 2010 and 2011. Since I upgraded it doesn't work. What's wrong with it? I have no idea how to fix this. All I want to do is substitute in the alternative expression for "u" into "equation". This should not be so hard!

syms u uBar uSigma
equation = diff(sym('a(u)'),u);
subs(equation,'u',uBar*uSigma)

Thanks for any help.

EDIT: The error I receive is

Error using mupadmex
Error in MuPAD command: The variable is invalid. [stdlib::diff]

Error in sym/subs>mupadsubs (line 146)
G = mupadmex('symobj::fullsubs',F.s,X2,Y2);

Error in sym/subs (line 119)
    G = mupadsubs(F,X,Y);

Error in MatlabExample (line 3)
subs(equation,'u',uBar*uSigma)

[Jan]

Please explain "it doesn't work" with any details. Solving a problem is easier than guessing it. Thanks.

[Oleg Komarov]

Error using mupadmex
Error in MuPAD command: The variable is invalid. [stdlib::diff]
Error in sym/subs>mupadsubs (line 139)
G = mupadmex('symobj::fullsubs',F.s,X2,Y2);
Error in sym/subs (line 124)
    G = mupadsubs(F,X,Y);

[Theodore]

I receive the above error.

I do not receive an error if I simply substitute in "uBar".

I also do not receive an error if I leave the function "a(u)" undifferentiated.

[Theodore]

Help!

[Oleg Komarov]

Have you tried submitting a support request?

[Walter Roberson]

What result is shown for "equation" ?

Does it change if you use

subs(equation, u, uBar*uSigma)

with the u not quoted ?