Error in S-function MDL_Derivatives

I'm creating a model using example mex file csfunc.c as a starting example. When I compile & execute I get a fatal error in MDL_Derivatives at time= 0. For this code as a test I'm using the input U(0) in one of the derivatives. This throws an error. If I delete the reference to U(0) and just leave the additions of 1E-3 the model works.

At the beginning of the S-Functiion I do have the define: #define U(element) (*uPtrs[element])

What could cause this fatal fault?

I also have the input port delclared as a feedthrough, with

if (!ssSetNumInputPorts(S, 1)) return;
end
    ssSetInputPortWidth(S, 0, 3);
    ssSetInputPortDirectFeedThrough(S, 0, 1);

#define MDL_DERIVATIVES  /* Change to #undef to remove function */
#if defined(MDL_DERIVATIVES)
  /* Function: mdlDerivatives =================================================
   * Abstract:
   *    In this function, you compute the S-function block's derivatives.
   *    The derivatives are placed in the derivative vector, ssGetdX(S).
   */
  static void mdlDerivatives(SimStruct *S)
  {
    real_T            *dx   = ssGetdX(S);
    real_T            *x    = ssGetContStates(S);
    InputRealPtrsType uPtrs = ssGetInputPortRealSignalPtrs(S,0);

    /* xdot= f(x,u) */
    dx[0]= 1E-3 + U(0);
    dx[1]= 1E-3; 
    dx[2]= 1E-3;
    dx[3]= 1E-3; 
    dx[4]= 1E-3;
      
  }
#endif /* MDL_DERIVATIVES *

Any help on this is greatly appreciated!

[Brian Tremaine]

I seemed to have fixed it by adding the line ssSetInputPortRequiredContiguous(S, 0, 0) to mdlInitializeSizes. The line was added after the setting up the inputs,

static void mdlInitializeSizes(SimStruct *S)
{
    ssSetNumSFcnParams(S, 5);  /* Number of expected parameters */
    if (ssGetNumSFcnParams(S) != ssGetSFcnParamsCount(S)) {
        /* Return if number of expected != number of actual parameters */
        return;
    }
    ssSetNumContStates(S, NUMSTATES);  // ia,ib,ic,wrad,theta
    ssSetNumDiscStates(S, 0);
   
    if (!ssSetNumInputPorts(S, 1)) return;
    ssSetInputPortWidth(S, 0, 3);
    ssSetInputPortDirectFeedThrough(S, 0, 1);
    /*
     * Set direct feedthrough flag (1=yes, 0=no).
     * A port has direct feedthrough if the input is used in either
     * the mdlOutputs or mdlGetTimeOfNextVarHit functions.
     */
    ssSetInputPortRequiredContiguous(S, 0, 0);