https://github.com/tobgu/pyrsistent/pull/284

--- pyrsistent-0.20.0/pvectorcmodule.c.orig
+++ pyrsistent-0.20.0/pvectorcmodule.c
@@ -1313,12 +1313,10 @@
 }
 
 static PyObject *PVectorEvolver_extend(PVectorEvolver *self, PyObject *args) {
-  PyObject *retVal = _PyList_Extend((PyListObject *)self->appendList, args);
-  if (retVal == NULL) {
+  if (PyList_SetSlice(self->appendList, PY_SSIZE_T_MAX, PY_SSIZE_T_MAX, args) < 0) {
     return NULL;
   }
 
-  Py_DECREF(retVal);
   Py_INCREF(self);
   return (PyObject*)self;
 }