Problem in calculation of the relic density

Dear MadDM developers,

I am trying to compute the relic density for a minimal DM model with MadDM. However, when I try to generate the output I got the following error message

Command "output " interrupted in sub-command:
"output" with error:
ValueError : String 1 Nc^1 T(2,1) cannot be simplified to a number!
Please report this bug on https://bugs.launchpad.net/mg5amcnlo
More information is found in 'MG5_debug'.
Please attach this file to your report.

Traceback (most recent call last):
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/interface/extended_cmd.py", line 1541, in onecmd
    return self.onecmd_orig(line, **opt)
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/interface/extended_cmd.py", line 1490, in onecmd_orig
    return func(arg, **opt)
  File "/home/ajueid/MG5_aMC_v2_9_3/PLUGIN/maddm/maddm_interface.py", line 1024, in do_output
    super(MadDM_interface, self).do_output(line)
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/interface/master_interface.py", line 299, in do_output
    self.cmd.do_output(self, line, *args, **opts)
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/interface/madgraph_interface.py", line 8053, in do_output
    self.export(nojpeg, main_file_name, group_processes, args)
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/interface/master_interface.py", line 313, in export
    return self.cmd.export(self, *args, **opts)
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/interface/madgraph_interface.py", line 8183, in export
    ndiags, cpu_time = generate_matrix_elements(self,group_processes)
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/interface/madgraph_interface.py", line 8139, in generate_matrix_elements
    subproc_groups.get_matrix_elements()])
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/iolibs/group_subprocs.py", line 515, in get_matrix_elements
    sum([group.get('matrix_elements') for group in self], []))
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/iolibs/group_subprocs.py", line 184, in get
    self.generate_matrix_elements()
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/iolibs/group_subprocs.py", line 229, in generate_matrix_elements
    generate_matrix_elements(amplitudes))
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/core/helas_objects.py", line 5870, in generate_matrix_elements
    compute_loop_nc=compute_loop_nc)
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/core/helas_objects.py", line 5683, in process_color
    col_matrix = color_amp.ColorMatrix(col_basis)
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/core/color_amp.py", line 564, in __init__
    self.build_matrix(Nc, Nc_power_min, Nc_power_max, is_symmetric=True)
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/core/color_amp.py", line 604, in build_matrix
    Nc)
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/core/color_amp.py", line 659, in create_new_entry
    result_fixed_Nc = result.set_Nc(Nc)
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/core/color_algebra.py", line 1053, in set_Nc
    return (sum([cs.set_Nc(Nc)[0] for cs in self if not cs.is_imaginary]),
  File "/home/ajueid/MG5_aMC_v2_9_3/madgraph/core/color_algebra.py", line 846, in set_Nc
    raise ValueError("String %s cannot be simplified to a number!" % str(self))

What could be the reason for this crash?

I can send you the UFO model file privately.

Thanks!

All the best,
Adil