How to plot isosurfaces in cylindrical polars?

I'm trying to plot isosurfaces of magnetic field strength for a field defined in cylindrical polars:

ModB = (nrpts,napts,nzpts);

For a given z I can plot contours with x=r*cos(theta) and y=r*sin(theta) (see below)

However if i take the same approach for the isosurfaces I get the following error message:

Error using griddedInterpolant The grid was created from grid vectors that were not strictly monotonic increasing.

Error in interp3 (line 141) F = griddedInterpolant(X, Y, Z, V, method,extrap);

Error in isonormals (line 76) n(:,1)=interp3(x, y, z, nx, verts(:,1), verts(:,2), verts(:,3));

Error in isosurface (line 124) isonormals(x,y,z,data, p);

I understand that this is because neither x or y are monotonic because of the nature of cos and sin, but I'm not sure how to get round the problem.

I can plot isosurfaces if r and theta are taken as the x and y axes but this doesn't represent the field properly (see below)

Any help would be much appreciated. Thanks, Nick.