Problem with function fminsearch

Hello everybody I'm trying to identify a parametre (K) which figure in this equation: P=P0+K*f, (P,P0, and f) are vectors I've done the below code but it does not work, thank you very much for your help

or j=1:(n/index)

if sv(j)<sip

 h=h+1;
   rc=j
   cpt=cpt+1;%Compteur du nombre de sous périodes pour identifier K
   ssptd=time(600*(rc-1):600*rc);
   Pv=P(600*(rc-1):600*rc);
   Fv=f(600*(rc-1):600*rc);
   p0=Pv-K*Fv;
   ss{1,rc}=ssptd;

end cpt end

 S = @(ki)(Sum(Pv-(P0+ki.*Fv)^2);
 res = fminsearch(S,100);;%ki parameter that I need to identify

message eroor: Error in fminsearch (line 190) fv(:,1) = funfcn(x,varargin{:});

thank you