diff --git a/beacon_chain/utils/vec_shuffle/src/lib.rs b/beacon_chain/utils/vec_shuffle/src/lib.rs
index 8c9de361e..b3d540174 100644
--- a/beacon_chain/utils/vec_shuffle/src/lib.rs
+++ b/beacon_chain/utils/vec_shuffle/src/lib.rs
@@ -47,6 +47,8 @@ mod tests {
     use std::fs::File;
     use std::io::prelude::*;
 
+    // TODO: update test vectors to use keccak instead of blake.
+    // https://github.com/sigp/lighthouse/issues/121
     #[test]
     #[should_panic]
     fn test_shuffling() {