index exceeded matrix dimensions please help

I get this error what should i do to correct this?

Index exceeds matrix dimensions.

Error in t31 (line 11)

F1=Fmag((N/2)+1:N);

my code

N=length(t);
Fshifted=fftshift(F);
Fmag=norm(Fshifted);

F1=Fmag((N/2)+1:N);
F2=Fmag(N/2:-1:1);

[per isakson]

((N/2)+1:N) is greater than 1

Fmag is a scalar

Thus the error

[Walter Roberson]

Do not use norm() to find the magnitude of the spectrum. Use abs() .