diff --git a/Modules/main.c b/Modules/main.c
index dc94a09..c1c4b45 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -99,6 +99,7 @@
 PYTHONHOME   : alternate <prefix> directory (or <prefix>%c<exec_prefix>).\n\
                The default module search path uses %s.\n\
 PYTHONCASEOK : ignore case in 'import' statements (Windows).\n\
+PYTHONIOENCODING: Encoding[:errors] used for stdin/stdout/stderr.\n\
 ";