diff --git a/src/librustdoc/lib.rs b/src/librustdoc/lib.rs
index 751f23010539..951bb27a9faf 100644
--- a/src/librustdoc/lib.rs
+++ b/src/librustdoc/lib.rs
@@ -275,7 +275,7 @@ fn opts() -> Vec<RustcOptGroup> {
                 "default-theme",
                 "Set the default theme. THEME should be the theme name, generally lowercase. \
                  If an unknown default theme is specified, the builtin default is used. \
-                 The set of themes, and the rustdoc built-in default is not stable.",
+                 The set of themes, and the rustdoc built-in default, are not stable.",
                 "THEME",
             )
         }),