support for mathutils GC wasn't complete since PyObject_GC_Track is supposed to be called once the fields are filled in.

source/blender/python/generic/mathutils_Matrix.c

@@ -1895,6 +1895,7 @@ PyObject *newMatrixObject_cb(PyObject *cb_user, int rowSize, int colSize, int cb
 		self->cb_user=			cb_user;
 		self->cb_type=			(unsigned char)cb_type;
 		self->cb_subtype=		(unsigned char)cb_subtype;
+		PyObject_GC_Track(self);
 	}
 	return (PyObject *) self;
 }