Pythia creating massless electron/positron pair

Dear experts,

I want to use MadEvent and Pythia to generate some events for an interacting dark matter model of the form e+ e- -> dark quark / anti dark quark. When I check the .lhe output from MadEvent only the 4 specified particles are present. When I then use Pythia to hadronize the quarks I can see that in the .hepmc file the first entries are the expected electrons and positrons but each is linked to a massless duplicate:

V -1 0 0 0 0 0 1 1 0
P 1 -11 0 0 5.1774999747831005e+00 5.1775000000000002e+00 5.1099999999999995e-04 4 0 0 -1 0
P 3 -11 0 0 5.1774999999725235e+00 5.1774999999725235e+00 0 21 0 0 -3 0
V -2 0 0 0 0 0 1 1 0
P 2 11 0 0 -5.1774999747831005e+00 5.1775000000000002e+00 5.1099999999999995e-04 4 0 0 -2 0
P 4 11 0 0 -5.1774999999725235e+00 5.1774999999725235e+00 0 21 0 0 -3 0

I couldn't find anything in the documentation as to why this happens and it also occurs when I generate electron/positron events that just use the SM and not my UFO. Is this an issue or just the way Pythia treats these kind of interactions?

Thanks in advance,
Tim