Showering an NLO process with Pythia8 : Empty hepmc file

Dear experts,

I get an empty hepmc file when I try to shower with Pythia8 in NLO run. MadGrapgh doesn't crash or comes out any error as seen below. The problem only occurs for NLO processes and it works perfectly well for any LO process. I don't know what the problem is.

INFO: The /Users/oozdal/hepwork/MG5_aMC_v2_7_2/pptoll_NLO/Events/run_03/events.lhe.gz file has been generated.

INFO: Events generated
reweight -from_cards
decay_events -from_cards
INFO: Preparing MCatNLO run
INFO: Using LHAPDF v6.2.1 interface for PDFs
INFO: Compiling MCatNLO for PYTHIA8...
INFO: ... done
INFO: Showering events...
INFO: (Running in /Users/oozdal/hepwork/MG5_aMC_v2_7_2/pptoll_NLO/MCatNLO/RUN_PYTHIA8_4)
INFO: Idle: 0, Running: 1, Completed: 0 [ current time: 10h52 ]
INFO: Idle: 0, Running: 0, Completed: 1 [ 1.4s ]
INFO: Idle: 0, Running: 0, Completed: 0 [ current time: 10h52 ]
INFO: The file /Users/oozdal/hepwork/MG5_aMC_v2_7_2/pptoll_NLO/Events/run_03/events_PYTHIA8_0.hepmc.gz has been generated.
It contains showered and hadronized events in the HEPMC format obtained showering the parton-level event file /Users/oozdal/hepwork/MG5_aMC_v2_7_2/pptoll_NLO/Events/run_03/events.lhe.gz with PYTHIA8
INFO: Run complete
INFO:
quit
INFO:

Thanks,
Ozer