diff --git a/src/atom.rs b/src/atom.rs
index 5a8aa7f..c11b3aa 100644
--- a/src/atom.rs
+++ b/src/atom.rs
@@ -336,14 +336,21 @@ impl<Static: StaticAtomSet> Ord for Atom<Static> {
 // over the one from &str.
 impl<Static: StaticAtomSet> Atom<Static> {
     fn from_mutated_str<F: FnOnce(&mut str)>(s: &str, f: F) -> Self {
-        let mut buffer = mem::MaybeUninit::<[u8; 64]>::uninit();
-        let buffer = unsafe { &mut *buffer.as_mut_ptr() };
-
-        if let Some(buffer_prefix) = buffer.get_mut(..s.len()) {
-            buffer_prefix.copy_from_slice(s.as_bytes());
-            let as_str = unsafe { ::std::str::from_utf8_unchecked_mut(buffer_prefix) };
-            f(as_str);
-            Atom::from(&*as_str)
+        const MAX_STACK_SIZE: usize = 64;
+        let mut buffer = mem::MaybeUninit::<[u8; MAX_STACK_SIZE]>::uninit();
+
+        if s.len() <= MAX_STACK_SIZE {
+            let buffer_ptr = buffer.as_mut_ptr() as *mut u8;
+            // SAFETY: `buffer_ptr` points to the `MaybeUninit` array.
+            // We use `copy_nonoverlapping` to write valid data into it,
+            // and then create a slice covering ONLY the initialized portion.
+            unsafe {
+                std::ptr::copy_nonoverlapping(s.as_ptr(), buffer_ptr, s.len());
+                let buffer_slice = slice::from_raw_parts_mut(buffer_ptr, s.len());
+                let as_str = ::std::str::from_utf8_unchecked_mut(buffer_slice);
+                f(as_str);
+                Atom::from(&*as_str)
+            }
         } else {
             let mut string = s.to_owned();
             f(&mut string);
diff --git a/tests/ub_test.rs b/tests/ub_test.rs
new file mode 100644
index 0000000..64c47bf
--- /dev/null
+++ b/tests/ub_test.rs
@@ -0,0 +1,8 @@
+use string_cache::DefaultAtom;
+
+#[test]
+fn test_to_ascii_uppercase() {
+    let s = DefaultAtom::from("hello");
+    let upper = s.to_ascii_uppercase();
+    assert_eq!(&*upper, "HELLO");
+}