Maxima: Differentiate sum at specific index-position



Hi All,

in Maxima 12.04.0 I have a sum
mysum : sum(u[i]^2, i, 1, N);

now I differentiate it
diff(mysum, u[i]);

now I specify a defined index i=A to differentiate it at
at(%, i=A);

Unfortunately maxima won't replace the u[i] in the sum that way.
How can I bring maxima to a result like
2*u[A]

Thanks for any help!

Regards,
Benni