diff --git a/library/core/src/hash/mod.rs b/library/core/src/hash/mod.rs
index 84bbf985e8be4..35b5eed48524f 100644
--- a/library/core/src/hash/mod.rs
+++ b/library/core/src/hash/mod.rs
@@ -549,8 +549,8 @@ pub trait Hasher {
     #[inline]
     #[unstable(feature = "hasher_prefixfree_extras", issue = "96762")]
     fn write_str(&mut self, s: &str) {
+        self.write_length_prefix(s.len());
         self.write(s.as_bytes());
-        self.write_u8(0xff);
     }
 }