Dealing with user-defined propagators

Hi,

I am using MG5_aMC_v2_1_0 with UFO output from FeynRules 2.0. The new version of UFO supplements a new file propagator.py. Following arxiv:1308.1668, if we have to use different form of some propagators, the attribute "propagator = propagators.V2" (say, for a massless vector boson) has to be provided in particles.py file. But when I run a process in MG5 with that particular massless particle as propagator, Madgraph session does not launch. But, when I give a process that do not have that propagator, it runs. (Of course, if I do not add that line at all, it runs as usual :)).

I am doing the following :
I have generated WEff_UFO model file using FeynRules.
Adding a line propagator = Prop.V2, say for example gluon. V2 is the standard form massless vector boson propagator given in propagator.py file. In particles.py, propagators are being imported as "import propagators as Prop".
Now generating p p > t t~, it runs into error. If, I do not use gluon as the mediating particle (QCD=0), it runs fine.

Can you please tell me what is going wrong or I am making some mistakes ? By the way, is there any way to provide the functional form of a propagator in the Feynrules itself ??

Thanks,
Saurabh Niyogi