3D waveguide, compute electromagnetic field

I want to view the electromagnetic field (in particular the norm of E) of 3D waveguide. I haven't experience in a 3D waveguide, I have simulate always a cross-section of waveguide. First of all I solved a cross-section in 2D model in COMSOL, after that I tried a 3D model, after drawing the waveguide I inserted two port (type of port is numeric, the excitation is On only one of this port).
In solver sequences created automatically by COMSOL 4 there is two study step, the first is Boundary mode analysis, the second is Frequency domain.

Problem:
The first study step compute rightly the field at boundary of waveguide, the eigenvalue is almost the same of that in 2D model, the second study step doesn't finish, there is an error. First appears a window:

"The following feature has encountered a problem:
Feature: Stationary 1 (sol1/s1)
Error: Undefined value found."

In the tab Error 1 there is:

"Undefined value found.
- Detail: Undefined value found in the equation residual vector.
There are 1 degrees of freedom giving NaN/Inf in the vector for the variable mod1.emw.S1x
at coordinates: (0,0,0), ...
There are 1 degrees of freedom giving NaN/Inf in the vector for the variable mod1.emw.S2x
at coordinates: (0,0,0), ..."


Where is the problem? Is there another method to solve my (I think and hope) simple problem (view the electromagnetic field in the entire 3D structure)?

Thanks in advance.
And sorry for my bad English.