Missing state variable at last step integration when using OutputFcn option

I'm trying to integrate a state-space model of a fluid-mechanical system. I call the integration like this

 options = odeset();
 [tt, state] = ode113(@fluidmech_system, tspan, state0, options, data);

and the @fluydmech_system functions calls another function

persistent sol
[sol] = computeVariables( t, X, data );

which solves an 8x8 algebraic implicit system using fsolve to retrieve some quantities needed to solve the state-space system. Everything works fine when I run the code like this, but I would like to save those quantities to the base workspace to plot them. Trying to use the OutputFcn option to do this, the code fails at the very last step of the integration with error

Index exceeds the number of array elements (0).

 Error in computeVariables (line 21)
if  X(3) >= acc.v0   % full accumulator

seems like the state vector X is not defined anymore. How can I solve the problem? The following is the code of the output function

function status = myOutputFcn(t, X, flag, data)

 [sol] = computeVariables( t, X, data );

 %solution stack
persistent sol_save
switch flag
    case 'init'
        sol_save = sol;
    case ''
        sol_save = [sol_save; sol];
    case 'done' % when it's done
        assignin('base','sol_save',sol_save); % get the data to the workspace.
end
status = 0;
end