diff --git a/src/core/collection/merge.rs b/src/core/collection/merge.rs
index 25e482c..e2a5fc5 100644
--- a/src/core/collection/merge.rs
+++ b/src/core/collection/merge.rs
@@ -22,7 +22,7 @@ impl<T: Ord> Merge for Vec<T> {
     }
 }
 
-impl<K: Hash + PartialEq + Eq, V: Merge> Merge for HashMap<K, V> {
+impl<K: Hash + PartialEq + Eq, T: Ord> Merge for HashMap<K, Vec<T>> {
     fn merge_in_place(&mut self, mut other: Self) {
         for (other_key, other_value) in other.drain() {
             if let Some(ref mut value) = self.get_mut(&other_key) {