diff --git a/components/util/cache.rs b/components/util/cache.rs
index 7a3dcfa980f..2efb6a4e781 100644
--- a/components/util/cache.rs
+++ b/components/util/cache.rs
@@ -12,7 +12,10 @@ use std::hash::{Hash, Hasher, SipHasher};
 use std::slice::Iter;
 
 
-pub struct HashCache<K, V> {
+pub struct HashCache<K, V>
+    where K: Clone + PartialEq + Eq + Hash,
+          V: Clone,
+{
     entries: HashMap<K, V, DefaultState<SipHasher>>,
 }