charm mass assigned to a gluon

Dear MG authors

I've produced events for a process pp>mu+mu-mu+mu-jj in sm-full with p = p b b~, j = j b b~. If you look closely at the LHEF file then, in the case when you have g c collision, mass of the charm quark gets sometimes wrongly assigned to a gluon:

10 0 9.80850E-09 0.9118760E+02 0.7562396E-02 0.1179816E+00
        -4 -1 0 0 0 501 0.00000000000E+00 0.00000000000E+00 0.58232961335E+03 0.58233200796E+03 0.00000000000 E+00 0. 1.
        21 -1 0 0 501 502 0.00000000000E+00 0.00000000000E+00 -0.32964103798E+03 0.32964103798E+03 0.16699999571 E+01 0. 1.
        23 2 1 2 0 0 -0.34116310171E+02 0.41762868029E+01 0.63666733963E+02 0.11769324077E+03 0.92827161528 E+02 0. 0.
        23 2 1 2 0 0 0.11713195275E+03 0.37032127569E+02 0.39014460077E+03 0.41803199310E+03 0.86294060732 E+02 0. 0.
       -13 1 3 3 0 0 -0.26521817197E+02 0.43314483600E+02 0.11567935331E+02 0.52090110086E+02 0.10565999895 E+00 0. 1.
        13 1 4 4 0 0 0.61376541368E+02 0.13472849986E+01 0.79673489919E+02 0.10058216128E+03 0.10565999895 E+00 0. 1.
       -13 1 4 4 0 0 0.55755411386E+02 0.35684842571E+02 0.31047111085E+03 0.31744983182E+03 0.10565999895 E+00 0. -1.
        13 1 3 3 0 0 -0.75944929739E+01 -0.39138196797E+02 0.52098798632E+02 0.65603130680E+02 0.10565999895 E+00 0. -1.
        21 1 1 2 503 502 0.14089538162E+02 -0.48742477536E+02 -0.25834000525E+03 0.26327533594E+03 0.00000000000 E+00 0. 1.

This does not happen when the first particle is a gluon and the second one is a charm quark

  10 0 9.80850E-09 0.9118760E+02 0.7562396E-02 0.1179816E+00
        21 -1 0 0 501 502 0.00000000000E+00 0.00000000000E+00 0.57713184380E+03 0.57713184380E+03 0.00000000000 E+00 0. 1.
        -4 -1 0 0 0 501 0.00000000000E+00 0.00000000000E+00 -0.18150597873E+03 0.18151366123E+03 0.16699999571 E+01 0. 1.
        23 2 1 2 0 0 0.94476631004E+01 -0.46571856602E+02 -0.62241370685E+02 0.12083558235E+03 0.92027460969 E+02 0. 0.
        23 2 1 2 0 0 0.10863093158E+03 0.16547183075E+03 0.22546129450E+03 0.31390336387E+03 0.92308832001 E+02 0. 0.
       -13 1 3 3 0 0 -0.29503441434E+02 0.10685016637E+02 -0.67533552962E+01 0.32097376977E+02 0.10565999895 E+00 0. 1.
        13 1 3 3 0 0 0.38951104534E+02 -0.57256873240E+02 -0.55488015388E+02 0.88738205376E+02 0.10565999895 E+00 0. -1.
       -13 1 4 4 0 0 0.50403201715E+02 0.14371820914E+03 0.13134949861E+03 0.20111715076E+03 0.10565999895 E+00 0. 1.
        13 1 4 4 0 0 0.58227729860E+02 0.21753621613E+02 0.94111795894E+02 0.11278621311E+03 0.10565999895 E+00 0. -1.
        21 1 1 2 503 502 0.14100756618E+02 -0.57945097206E+02 0.13819325415E+03 0.15051193016E+03 0.00000000000 E+00 0. 1.
        -4 1 1 2 0 503 -0.13217935129E+03 -0.60954876946E+02 0.94212687110E+02 0.17339462865E+03 0.16699999571 E+01 0. 1.

best regards,
Wojciech