symbolic calculation - subs does not work after diff

In the following code I create a symbolc expression. I want to differentiate it and after that I want to evaluate it at specific datapoints. The evaluation works before the diff command but not afterwards. In other examples this works well but not in the one shown below. Any ideas?

syms x
A=1;
B=2;
D=1;
C=3;
E=A+(B*C^2)./(B^2-x.^2-1i.*C.*x);
n=sqrt((sqrt(real(E).^2+imag(E).^2)+real(E))./2);
y=10:1:300;
nN=subs(n,x,y);
plot(y,nN)     
dn=diff(n);
dnN=subs(dn,x,y)
plot(y,dnN)

Thanks, Dirk

Answer 1

[Andrei Bobrov]

syms x real
A=1;
B=2;
D=1;
C=3;
E=A+(B*C^2)./(B^2-x.^2-1i.*C.*x);
n=sqrt((sqrt(real(E).^2+imag(E).^2)+real(E))./2);
y=10:1:300;
nN=subs(n,x,y);
plot(y,nN)
dn=diff(n);
dnN=subs(dn,x,y)
plot(y,dnN)