diff --git a/src/Contrl/AccSimulator.f90 b/src/Contrl/AccSimulator.f90
index 4b71bc2..c64215c 100644
--- a/src/Contrl/AccSimulator.f90
+++ b/src/Contrl/AccSimulator.f90
@@ -1459,6 +1459,9 @@ subroutine run_AccSimulator()
           nptottmp = sum(Np)
           !exit if the beam is outside the beamline
           if(distance.gt.blnLength .or. distance.gt.tstop .or. nptottmp.lt.1) then
+            if(nptottmp.lt.1) then
+              print*,"Warning: total # of particles is less than 1!! ",nptottmp
+            endif
             exit
           endif