MATLAB: FMINUNC cannot continue.

fminunc

Hi, I'm trying to learning fminunc, but it gives errors: 'Insufficient number of input parameters' and 'Failure in initial user-supplied objective function evaluation. FMINUNC cannot continue.'
The cose is as following. I have saved the function Pun as a separate file.
Help please?
function f = Pun(x,lamda)
f = x(1)^2 + lamda * x(2)^2;
end
x = [0.5;0.5];
lamda = 0.5;
fminunc('Pun',x,lamda)