surface plotting with complex function

why with this code the surface ploting does not happen.? Where I am wrong with this code. With this code if write 'plot3(Th,Al,mag', then some 3d image is apperaing. But surface plotting is not happen. Pl help me.

f(theta,alpha)=1i*int(r,phi,0,2*pi);  %% f is acomplex function

theta =linspace(0.1,4,20);
alpha =linspace(0.1,4,20);
[Th,Al] = ndgrid(theta, alpha);
F = f(Th,Al);
mag= abs(F);
surf(Th,Al,mag)
%grid on

[darova]

Do you have original formula/expression for f function?

Answer 1

[Thiago Henrique Gomes Lobato]

Maybe you need to add a shading to properly visualize the result?

surf(Th,Al,mag)
shading interp

[AVM]

But still error is coming. Only 2-D axes are showing and no image. The error are

Error using matlab.graphics.chart.primitive.Surface/set
Invalid parameter/value pair arguments.

Error in prlmatrix (line 36)
surf(Th,Al,F)