How do I fix this error: "Error using minus Matrix dimensions must agree"?

Here is my script:

 function f = objectiveL2(p, pIndex, data, trainingIndex, kinesinName, lambda)

    if length(p) ~= size(pIndex,1)
        error('p and pIndex must have the same length');
    end

    p00 = InitialParameterValues(kinesinName);
    p0 = InitialParameterValues(kinesinName);
    for i = 1:size(pIndex,1)
        for j = 1:size(pIndex,2)
            if pIndex(i,j) ~= 0
                p00(pIndex(i,j)) = p(i)*p00(pIndex(i,j));
            end
        end
    end

    for i = 1:size(data,1)
        if strcmp(data(i,1).kinesinName,kinesinName)
            kinesinIndex = i;
            break;
        end
    end

    f = 0;
    for i = 1:length(trainingIndex)
        % Define initial conditions for the model
        x0 = zeros(12,1);
        x0(1)       = data(kinesinIndex,trainingIndex(i)).kinesinConc;     % Kinesin
        x0(2)       = data(kinesinIndex,trainingIndex(i)).MTConc;          % MT
        x0(8)       = data(kinesinIndex,trainingIndex(i)).mantATPConc;     % mantATP

        tspan       = [0;data(kinesinIndex,trainingIndex(i)).time];

        [~,x]       = ode45(@(t,x,p) SteppingModel_no78(1,x,p00), tspan, x0);
        y_model     = p00(15)*(x(2:end,9)+x(2:end,10));
        y_data      = data(kinesinIndex,trainingIndex(i)).fluorescence - 1;

        f           = f + sum((y_model - y_data).^2)/length(y_data);
    end

    f = f + lambda*sum(((p0(1:14) - p00(1:14))./p0(1:14)).^2);

 end

Here is the error I get

 Error using  - 
 Matrix dimensions must agree.

 Error in objectiveL2 (line 38)
        f           = f + sum((y_model - y_data).^2)/length(y_data);

 Error in @(p)objectiveL2(p,pIndex,data,trainingIndex,kinesinName,0.0000000000001)

 Error in fmincon (line 534)
      initVals.f = feval(funfcn{3},X,varargin{:});

 Error in main (line 60)
 [pFit, fval, flag] = fmincon(@(p) objectiveL2(p, pIndex, data, trainingIndex, kinesinName, 0.0000000000001), ...

 Caused by:
    Failure in initial user-supplied objective function evaluation. FMINCON cannot continue.

I know where the error is occurring (line 38), but I really don't know how to fix it. It looks correct to me. Any help would be greatly appreciated.

Thanks!

[Geoff Hayes]

Sean - what are the dimensions of y_model and y_data? Since you are subtracting one from the other, they must have the same number of rows and the same number of columns.

[Walter Roberson]

Where is kinesinIndex being defined? Is it a function, or is it being defined in an outer function that this one is nested inside?