Propagation in PEC waveguide

Hi,

I was trying to simulate a rectangular waveguide (side=500nm) with PEC sidewalls. I am using a port boundary condition to excite a TE01 wave from the top and use another port condition at the bottom of the waveguide. I find that the wave cut-off is exactly at 1000nm as it should be, and for wavelengths higher than 1000nm the wave is getting attenuated. However, for wavelengths below the cut-off I am getting and error as:

Attempt to evaluate real square root of negative number.
- Function: sqrt
Failed to evaluate variable.
- Variable: mod1.emw1.alphaport_2
- Defined as: sqrt((((1428571.428571428638*pi)^2)+(-(mod1.emw1.k^2))))
Failed to evaluate expression.
- Expression: imag(mod1.emw1.alphaport_2)

I am unable to figure out this as I expect waves to propagate below cut-off wavelength as well.

Any comments/suggestions would be greatly appreciated. I am attaching my model file with the post.

Thanks,
Naresh