About user define propagator

Dear MG5 team,

I will like to use a user defined propagator for scalar.

After added the propagator in propagators.py, I added "propagator=Prop.S2" in particles.py.

When I run it I got this error message after "output" :

Command "output" interrupted with error:
TypeError : 'LexToken' object has no attribute '__getitem__'
Please report this bug on https://bugs.launchpad.net/mg5amcnlo
More information is found in 'MG5_debug'.
Please attach this file to your report.

But if I directly use the same definition of S2 to replace original propagator S, there is no error message.

For I want to use original propagator for scalar1 and new propagator for scalar2,
may I ask is there any other files I need to modify to add new propagator?

Thanks.

Best,
Jung