Problem with functional derivative

Dear,

I am trying evaluate variational derivative of the form:

delta(F)/delta(e1)

F is the functional and e1, e2 and e3 are strain measures.

I used the following code, but get a zero result. Could you please help if I am going wrong.

syms e1(x,y) e2(x,y) e3(x,y) a1=1; a2=1; f = a1*diff(e1,x,x)+diff(e1,y,y)+a2*(diff(e2,x,x)+diff(e2,y,y)+diff(e3,x,x)+diff(e3,y,y)); D = functionalDerivative(f,e1)

Thank you, SS