diff --git a/src/lib.rs b/src/lib.rs
index 02b50f6..bca77d9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -55,6 +55,7 @@ pub trait Group:
         rustc_serialize::Decodable +
         'static +
         Send +
+        Sync +
         Copy +
         Clone +
         PartialEq +