Why do I get "Assignment has more non-singleton rhs dimensions than non-singleton subscripts" by using fminsearch?

I would like to find the arg minimum of the defined 'fun'. But I get the following error:

Assignment has more non-singleton rhs dimensions than non-singleton subscripts

Error in fminsearch (line 200)

fv(:,1) = funfcn(x,varargin{:});

Error in HowardNumeric (line 46)

fminsearch(fun,x0)

fun = @(y) y.*dXXu + f';
x0 = zeros(length(x),1);
fminsearch(fun,x0)

While: size(dXXu)=size(x0)=size(f')=[10, 1].

What am I missing here? Thank you.

[KSSV]

What is x? Show us the full code.