diff --git a/Modules/pyexpat.c b/Modules/pyexpat.c
index 3311093..30c420d 100644
--- a/Modules/pyexpat.c
+++ b/Modules/pyexpat.c
@@ -969,7 +969,7 @@
     xmlparseobject *new_parser;
     int i;
 
-    if (!PyArg_ParseTuple(args, "s|s:ExternalEntityParserCreate",
+    if (!PyArg_ParseTuple(args, "z|s:ExternalEntityParserCreate",
                           &context, &encoding)) {
         return NULL;
     }