Subscript indices must either be real positive integers or logicals

Hello everyone!

I am getting indices error in this for loop when computing Vx(m,n), although they are real positive integers, what could be the problem?

nGridX = 100;

nGridY = 100;

xVals = [-0.5; 1.5];

yVals = [-0.5; 0.5];

%Streamlines parameters

stepsize = 0.01;

maxVert = nGridX * nGridY * 10;

slPct = 30;

Ysl = linspace (yVals(1), yVals(2),floor((slPct/100)*nGridY))';

%Generate the grid points

Xgrid = linspace(xVals(1),xVals(2),nGridX)';

Ygrid = linspace(yVals(1),yVals(2),nGridY)';

[XX,YY] = meshgrid(Xgrid,Ygrid);

%Initizalize

Vx = zeros(nGridX,nGridY);

Vy = zeros(nGridX,nGridY);

for m = 1:nGridX

for n = 1:nGridY

XP = XX(m,n);

YP = YY(m,n);

[Mx, My] = Streamline_function1(XP, YP, x, y, phi,s);

[in, on] = inpolygon (XP, YP, x, y);

if (in == 1 || on == 1)

Vx(m,n) = 0;

Vy(m,n) = 0;

else

Vx(m,n) = V_inf*cosd(AoA_deg) + sum(lambda.*Mx./(2*pi));

Vy(m,n) = V_inf*sind(AoA_deg) + sum(lambda.*My./(2*pi));

end

end

end