discrete_constraint_estimatestruct assert Graph contains a cycle

[locusxt]

I use discrete_constraint_estimatestruct to learn structure, it failed and return
Traceback (most recent call last):
File "fs.py", line 77, in
result = learner.discrete_constraint_estimatestruct(bn_data)
File "/Users/zhuting/Projects/PyWorkspace/BayesianNet/venv/lib/python2.7/site-packages/libpgm/pgmlearner.py", line 590, in discrete_constraint_estimatestruct
pdag.toporder()
File "/Users/zhuting/Projects/PyWorkspace/BayesianNet/venv/lib/python2.7/site-packages/libpgm/graphskeleton.py", line 146, in toporder
assert (not Ecopy), ("Graph contains a cycle", Ecopy)
AssertionError: ('Graph contains a cycle', [[164, 21], [21, 58], [21, 16], [16, 11], [58, 164], [11, 67], [16, 2], [164, 2]])