When attempting to cast to a user defined type, PyArray_GetCast looks up the cast function in the dictionary but doesn't check if the entry exists. This causes segfaults. Here's a patch. Geoffrey diff --git a/numpy/core/src/multiarray/convert_datatype.c b/numpy/core/src/multiarray/convert_datatype.c index 818d558..4b8f38b 100644 --- a/numpy/core/src/multiarray/convert_datatype.c +++ b/numpy/core/src/multiarray/convert_datatype.c @@ -81,7 +81,7 @@ PyArray_GetCastFunc(PyArray_Descr *descr, int type_num) key = PyInt_FromLong(type_num); cobj = PyDict_GetItem(obj, key); Py_DECREF(key); - if (NpyCapsule_Check(cobj)) { + if (cobj && NpyCapsule_Check(cobj)) { castfunc = NpyCapsule_AsVoidPtr(cobj); } }