diff --git a/src/libcollections/str.rs b/src/libcollections/str.rs
index 8d5d76f9598f..e5aa377b2754 100644
--- a/src/libcollections/str.rs
+++ b/src/libcollections/str.rs
@@ -202,7 +202,7 @@ fn next(&mut self) -> Option<char> {
                 let buffer = &mut self.buffer;
                 let sorted = &mut self.sorted;
                 {
-                    let callback = |d| {
+                    let callback = |&mut: d| {
                         let class =
                             unicode::char::canonical_combining_class(d);
                         if class == 0 && !*sorted {