diff --git a/src/mono_hash_map.rs b/src/mono_hash_map.rs
index 42ac42544f00..278bbd9cf2b1 100644
--- a/src/mono_hash_map.rs
+++ b/src/mono_hash_map.rs
@@ -18,7 +18,7 @@ use crate::AllocMap;
 pub struct MonoHashMap<K: Hash + Eq, V>(RefCell<FxHashMap<K, Box<V>>>);
 
 impl<K: Hash + Eq, V> MonoHashMap<K, V> {
-    pub fn values<T>(&self, f: impl FnOnce(&mut Iterator<Item=&V>) -> T) -> T {
+    pub fn values<T>(&self, f: impl FnOnce(&mut dyn Iterator<Item=&V>) -> T) -> T {
         f(&mut self.0.borrow().values().map(|v| &**v))
     }
 }