diff -r 9a639a33ad1f -r 1f8c3fadbb8e mercurial/cext/parsers.c
--- a/mercurial/cext/parsers.c	Sat Mar 03 05:58:41 2018 -0500
+++ b/mercurial/cext/parsers.c	Sat Mar 03 06:08:22 2018 -0500
@@ -254,7 +254,7 @@
 		goto quit;
 	}
 
-	parents = Py_BuildValue("s#s#", str, 20, str + 20, 20);
+	parents = Py_BuildValue(PY23("s#s#", "y#y#"), str, 20, str + 20, 20);
 	if (!parents)
 		goto quit;