stringio doesn't have an encoding
diff --git a/Modules/_stringio.c b/Modules/_stringio.c
index a88fcb7..d75a604 100644
--- a/Modules/_stringio.c
+++ b/Modules/_stringio.c
@@ -665,7 +665,7 @@
 {
     CHECK_INITIALIZED(self);
     CHECK_CLOSED(self);
-    return PyUnicode_FromString("utf-8");
+    Py_RETURN_NONE;
 }
 
 static PyObject *