diff -r 3264d58e8b06 -r 03332e5f67e9 mercurial/cext/revlog.c
--- a/mercurial/cext/revlog.c	Sat Jul 18 18:19:14 2020 +0900
+++ b/mercurial/cext/revlog.c	Sat Jul 18 18:21:26 2020 +0900
@@ -867,6 +867,10 @@
 			pyphase = phasesets[3];
 			break;
 		default:
+			/* this should never happen since the phase number is
+			 * specified by this function. */
+			PyErr_SetString(PyExc_SystemError,
+			                "bad phase number in internal list");
 			goto release;
 		}
 		pyrev = PyInt_FromLong(rev);