diff --git a/src/librustc/driver/session.rs b/src/librustc/driver/session.rs
index 3fdf4126c15b..ba58b78a7927 100644
--- a/src/librustc/driver/session.rs
+++ b/src/librustc/driver/session.rs
@@ -234,7 +234,7 @@ pub impl Session_ {
                  msg: &str) {
         let level = lint::get_lint_settings_level(
             self.lint_settings, lint_mode, expr_id, item_id);
-        let msg = fmt!("%s [-W%s]", msg, lint::get_lint_name(lint_mode));
+        let msg = fmt!("%s [-W %s]", msg, lint::get_lint_name(lint_mode));
         self.span_lint_level(level, span, msg);
     }
     fn next_node_id(@self) -> ast::node_id {