diff --git a/library/core/src/num/mod.rs b/library/core/src/num/mod.rs
index d9cac8b4259..44918a711d4 100644
--- a/library/core/src/num/mod.rs
+++ b/library/core/src/num/mod.rs
@@ -282,7 +282,7 @@ pub const fn is_ascii(&self) -> bool {
     /// ```
     ///
     /// [`make_ascii_uppercase`]: Self::make_ascii_uppercase
-    #[must_use = "to uppercase the value in-place, use `make_ascii_lowercase()`"]
+    #[must_use = "to uppercase the value in-place, use `make_ascii_uppercase()`"]
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.52.0")]
     #[inline]