Port extensio with HP 8510B

I want to do some S-parameters measurement with an HP 8510B using the port extension capabilities. In particular I want to extend the reference plane on one port because I need to connect a straight waveguide assembly. I tested the port extension using a short at the end of the inserted assembly, but I don't get a flat phase response after entering the assembly length.
I guess it's because the VNA assumes a linear phase delay, whilst in the waveguide is not linear. How can I correct the phase?
regards

grendhell