diff -r af090796cb33 -r b970418bbafe mercurial/parsers.c
--- a/mercurial/parsers.c	Tue Aug 18 16:39:26 2015 -0400
+++ b/mercurial/parsers.c	Tue Aug 18 16:40:10 2015 -0400
@@ -481,8 +481,11 @@
 	len = readlen;
 
 	/* read parents */
-	if (len < 40)
+	if (len < 40) {
+		PyErr_SetString(
+			PyExc_ValueError, "too little data for parents");
 		goto quit;
+	}
 
 	parents = Py_BuildValue("s#s#", str, 20, str + 20, 20);
 	if (!parents)