Debugger stuck in NumPy call

Mac OS 10.10.5, Python 2.7.10, NumPy 1.8.0rc1

I can debug to this point, but then seem stuck in endless loop once I step over to this line:

  z_grid = np.apply_along_axis(phi, 1, x_grid)

And upon forcing a quit I get this on the console:

  /usr/bin/python /Applications/PyCharm.app/Contents/helpers/pydev/pydevd.py --multiproc --qt-support --client 127.0.0.1 --port 52315 --file        /PycharmProjects/test.py
 pydev debugger: process 81088 is connecting

 Connected to pydev debugger (build 162.1967.10)
 Backend MacOSX is interactive backend. Turning interactive mode on.
 Traceback (most recent call last):
 File "_pydevd_bundle/pydevd_cython.pyx", line 472, in _pydevd_bundle.pydevd_cython.PyDBFrame.manage_return_values.get_func_name  (_pydevd_bundle/pydevd_cython.c:8198)
 cls_name = get_clsname_for_code(code_obj, frame)
 File "/Applications/PyCharm.app/Contents/helpers/pydev/_pydevd_bundle/pydevd_utils.py", line 133, in get_clsname_for_code
 funcs = [f for f in gc.get_referrers(code)
 KeyboardInterrupt

 Process finished with exit code 137 (interrupted by signal 9: SIGKILL)

 Any advice appreciated.