Functions mbstreamwriter_dealloc() and mbstreamreader_dealloc() of Modules/cjkcodecs/multibytecodec.c uses Py_DECREF() to free stream attribute memory, but this attribute may be NULL if MultibyteCodec or MultibyteStreamReader constructor fails. Simple fix: use Py_XDECREF(). Example: >>> import _multibytecodec >>> _multibytecodec.MultibyteStreamReader(None) Erreur de segmentation (core dumped)