S-function problem

When trying to run my program it says: An error occurred while running the simulation and the simulation was terminated Caused by: State derivatives returned by S-function 'EJEMPLO3' in 'ejemplo3SIMULINK/S-Function' during flag=1 call must be a real vector of length 4 This is the code: function [sys,x0,str,ts] = EJEMPLO3(t,y,u,flag,xo) switch flag,

%%%%%%%%%%%%%%%%%%
% Initialization %
%%%%%%%%%%%%%%%%%%
case 0,
  [sys,x0,str,ts] = mdlInitializeSizes(xo);

%%%%%%%%%%%%%%%
% Derivatives %
%%%%%%%%%%%%%%%
case 1,
  sys = react(t,y,u,0);

%%%%%%%%%%%
% Outputs %
%%%%%%%%%%%
case 3,
  sys = react(t,y,u,1);

%%%%%%%%%%%%%%%%%%%
% Unhandled flags %
%%%%%%%%%%%%%%%%%%%
case { 2, 4, 9 },
  sys = [];

%%%%%%%%%%%%%%%%%%%%
% Unexpected flags %
%%%%%%%%%%%%%%%%%%%%
otherwise
  error(['Unhandled flag = ',num2str(flag)]);

end % end csfunc

% %============================================================================= % mdlInitializeSizes % Return the sizes, initial conditions, and sample times for the S-function. %=============================================================================

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

sizes = simsizes; sizes.NumContStates = 4; %variables dependientes sizes.NumDiscStates = 0; sizes.NumOutputs = 4; %todas las salidas sizes.NumInputs = 2; %variables independietes o entradas sizes.DirFeedthrough = 1; sizes.NumSampleTimes = 1;

sys = simsizes(sizes); x0 = [xo]; str = []; ts = [0 0];

% end mdlInitializeSizes % %============================================================================= % react % Regresa los estados y par?metros necesarios %=============================================================================

function sys=react(t,y,u,resp) %parametros del sistema v1=150; v2=80; fr=42; k1=9.5537; k2=3.52;

if resp == 0 %MODELO dxdy=[(u(1)*u(2)/v1)+(fr*y(3)/v1)-((u(1)+fr)*y(1)/v1)-(k1*y(1)^2)+(2*k2*y(2));... (fr*y(4)/v1)-((u(1)+fr)*y(2)/v1)+((k1*y(2)^2)/2)-(k2*y(2));... ((u(1)+fr)*y(1)/v2)-((u(1)+fr)*y(3)/v2)-(k1*y(3)^2)+(2*k2*y(4));... ((u(1)+fr)*y(2)/v2)-((u(1)+fr)*y(4)/v2)+((k1*y(3)^2)/2)-(k2*y(4))]; sys = dxdy;

else

%SALIDAS

    sys =[y(1) y(2) y(3) y(4)];
end