diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 5306f2c9689b..78aabf86e300 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -2082,7 +2082,7 @@ pub fn build_session_options_and_crate_config(
                         error_format,
                         &format!(
                             "optimization level needs to be \
-                             between 0-3 (instead was `{}`)",
+                             between 0-3, s or z (instead was `{}`)",
                             arg
                         ),
                     );