differential of a series

Hi there,

I am a starter with a perhaps quite simple question.  I want to take 
derivative of a sum of series, say, f:sum(c[i]*K[i],i,1,n).   When I 
typed diff(f,c[i]), I got sum(K[i],i,1,n).

I also tried diff(f,c[1]), but got zero as the answer (please see 
below).  Is there any way to get the right answer in maxima?

Many thanks for your help!  x.w.

(%i1) f:sum(a[i]*K[i],i,1,n);
                                                                      n
                                                                     ====
                                                                     \
(%o1) >    a  K
                                                                     
/      i  i
                                                                     ====
                                                                     i = 1
(%i2) diff(f,a[i]);
                                                                       n
                                                                      ====
                                                                      \
(%o2) >    K
                                                                      
/      i
                                                                      ====
                                                                      i = 1
(%i3) diff(f,a[1]);
(%o3)                                                                                 
0