diff --git a/library/core/src/fmt/rt.rs b/library/core/src/fmt/rt.rs
index a431ee54d9508..2c1a767691b8b 100644
--- a/library/core/src/fmt/rt.rs
+++ b/library/core/src/fmt/rt.rs
@@ -1,5 +1,5 @@
 #![allow(missing_debug_implementations)]
-#![unstable(feature = "fmt_internals", issue = "none")]
+#![unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
 
 //! These are the lang items used by format_args!().