diff --git a/lib/services/address/encoding.js b/lib/services/address/encoding.js
index ad4b55eb..8ebce51c 100644
--- a/lib/services/address/encoding.js
+++ b/lib/services/address/encoding.js
@@ -24,7 +24,7 @@ exports.encodeMempoolAddressIndexKey = function(hashBuffer, hashTypeBuffer) {
     hashBuffer,
     hashTypeBuffer,
   ]);
-  return key;
+  return key.toString('binary');
 };