matlab error: capricious duty slight x quarrel arguments form 'double'

i'm new user matlab, greatfully help:

i have following formula an .m file:

function f = divrat(w, c)
s=sqrt(diag(diag(c)));
s=diag(s);
f=sqrt(w'*c*w)/(w'*s);

i have stored record (divrat.m) normal matlab path, therefore i'm presumption matlab review duty it's starting duty therefore should accessible use.

however, i type

>> divrat(w, c)

i following error

??? capricious duty slight 'divrat' quarrel arguments form 'double'.

what blunder summary explanation me do, i can't any blunder formula duty call?