why do i get Error in 'untitled1/DDE Source/S-Function' while executing MATLAB S-function 'sfunddi', flag = 0 (initialize), at start of simulation. Caused by: DDE initialization failed.

function [sys,x0,str,ts] = sfunddi(t,x,u,flag,service,topic,item) %SFUNDDI Simulink DDE source.

switch flag

case 0;
 [sys,x0,str,ts]=mdlInitializeSizes(service,topic);

case 2;
  sys = mdlUpdate(t,x,u);

case 3;
  sys = mdlOutputs(t,x,u,item);

case 9;
  sys = mdlTerminate(t,x,u);

otherwise;
  sys=[];

end

function [sys,x0,str,ts]=mdlInitializeSizes(service,topic)

sizes = simsizes;

sizes.NumContStates = 0; sizes.NumDiscStates = 1; sizes.NumOutputs = 1; sizes.NumInputs = 0; sizes.DirFeedthrough = 0; sizes.NumSampleTimes = 1;

sys = simsizes(sizes);

str = []; ts = [-1 0]; % Inherited sample time

x0 = ddeinit(service,topic); if (x0==0) error('DDE initialization failed.'); end;

function sys = mdlUpdate(t,x,u)

sys = x;

function sys = mdlOutputs(t,x,u,item)

sys = ddereq(x, item);

function sys = mdlTerminate(t,x,u)

ddeterm(x); sys = [];