Error with GA: Conversion to double from struct is not possible.

I’m trying to optimize stiffness of a spring in a train suspension system by GA. I get the value of fitness function from “To workspace” block, from a simulink model named “R1” which do a co-simulation by SIMPACK for getting the final result. As I run GA the following error occurs:

??? The following error occurred converting from struct to double:
Error using ==> double
Conversion to double from struct is not possible.

Error in ==> fcnvectorizer at 14
            y(i,:) = feval(fun,(pop(i,:)));

Error in ==> makeState at 47
        Score =
        fcnvectorizer(state.Population(initScoreProvided+1:end,:),FitnessFcn,1,options.SerialUserFcn);

Error in ==> gaunc at 41
state = makeState(GenomeLength,FitnessFcn,Iterate,output.problemtype,options);

Error in ==> ga at 291
        [x,fval,exitFlag,output,population,scores] = gaunc(FitnessFcn,nvars, ...

Caused by:
    Failure in user-supplied fitness function evaluation. GA cannot continue.

28          rethrow(userFcn_ME)

I don’t know how can I eliminate this weird error? I’m pressed with time unfortunately. Here is my code:

function [R]=Ride(d1)

%Calling the Function to Update Variables
'SubvarUpd';

%Running GA for Simulink Block
options = simset('SrcWorkspace','current');
R1
sim('R1',[],options)
R=RideComfortindex

And “SubvarUpd” function is:

function SubvarUpd(d1)
  kpx=d1;

    fileID = fopen('F:\MST\Optimization\1\mysubvar.subvar','w');                       %This is the address of SubVar file 

    % Longitudinal Stiffness subvar:  

    fprintf(fileID,'%s','subvar($_Rnom, str = ''');
    fprintf(fileID,'%d',kpx);

[Geoff Hayes]

Katie - what is your fitness function because it isn't clear from the above code which (of the functions that you provided) it is. The error is telling you that there is a problem with it, so you may want to take a closer look at this function.