S-function error

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hi guys when i try to use the follow s-function in a simulink block, i obtain this error:

Error in 'NARMA/S-Function' while executing MATLAB S-function 'NARMASYSTEM', flag = 0 (initialize), at start of simulation.Output argument "x0" (and maybe others) not assigned during call to "F:\Ms control\Adaptive control\Project\MATLAB\NARMASYSTEM.m>NARMASYSTEM".

This is the function code:

function [sys,x0,str,ts] = NARMASYSTEM(t,x,u,flag)
%M-file S-function for defining a discrete system.
% This S-function implements discrete equations
% Generate a discrete linear system:
a1=0.2;
a2=0.1;
g1=1;
g2=0.2;%=-0.2
b1=0.3;
b2=-0.6;
switch flag,
  case 0
    sys = mdlInitializeSizes(a1,a2,g1,g2,b1,b2); % Initialization
    case 2
      sys = mdlUpdate(t,x,u,a1,a2,g1,g2,b1,b2); % Update discrete states
    case 3
      sys = mdlOutputs(t,x,u,a1,a2,g1,g2,b1,b2); % Calculate outputs
    case {1, 4, 9} % Unused flags
      sys = [];
    otherwise
      error(['unhandled flag = ',num2str(flag)]); % Error handling
  end
  % End of dsfunc.
%==============================================================
% Initialization
%==============================================================
function [sys,x0,str,ts] = mdlInitializeSizes(a1,a2,g1,g2,b1,b2)
% Call simsizes for a sizes structure, fill it in, and convert it 
% to a sizes array.
sizes = simsizes;
sizes.NumContStates  = 0;
sizes.NumDiscStates  = 2;
sizes.NumOutputs     = 2;
sizes.NumInputs      = 2;
sizes.DirFeedthrough = 0; % Matrix D is non-empty.
sizes.NumSampleTimes = 0.05;
sys = simsizes(sizes); 
x0  = [0 1];   % Initialize the discrete states.
str = [];          % Set str to an empty matrix.
ts  = [1 0];       % sample time: [period, offset]
% End of mdlInitializeSizes.
%==============================================================
% Update the discrete states
%==============================================================
function sys = mdlUpdates(t,x,u,a1,a2,g1,g2,b1,b2)
sys(1) =a1*x(1)*cos(x(1))+a2*(x(1)^2)/(1+x(1)^2)+g1*x(2) ;
sys(2)=b1*sin(x(1))+b2*(x(1)^3)/(2+x(1)^2)+g2*u(1)+u(2);
% End of mdlUpdate.
%==============================================================
% Calculate outputs
%==============================================================
function sys = mdlOutputs(t,x,u,a1,a2,g1,g2,b1,b2)
sys(1) =x(1);
sys(2)=x(2);
% End of mdlOutputs.