assertion error: assert closest_shell_distance > 0

[Jintram]

A simple script simulating A+B <-> C crashed using the following parameters:

sigma=1e-10; D=5e-09; world_size=1e-05; k1=2.51327412287e-19; Na=5; Nb = 5; matrix_size=1

And also with the following parameters:

sigma=1e-07; D=5e-09; world_size=0.001;  k1=2.51327412287e-18; N=5; matrix_size=1

This is the crash report:

  File "/storage1/wehrens/myfork_egfrd/egfrd.py", line 299, in step
    f(self, event.data)
  File "/storage1/wehrens/myfork_egfrd/egfrd.py", line 792, in fire_single
    obj = self.form_pair_or_multi(single, burst)
  File "/storage1/wehrens/myfork_egfrd/egfrd.py", line 1175, in form_pair_or_multi
    neighbors[0], neighbors[1:])
  File "/storage1/wehrens/myfork_egfrd/egfrd.py", line 1283, in form_pair
    assert closest_shell_distance > 0
AssertionError