diff --git a/lib/crypto/OlmDevice.js b/lib/crypto/OlmDevice.js
index b3be020c3..5feb8016f 100644
--- a/lib/crypto/OlmDevice.js
+++ b/lib/crypto/OlmDevice.js
@@ -663,8 +663,8 @@ OlmDevice.prototype.decryptGroupMessage = function(
             var messageIndexKey = senderKey + "|" + sessionId + "|" + res.message_index;
             if (messageIndexKey in self._inboundGroupSessionMessageIndexes) {
                 throw new Error(
-                    "Duplicate message index, possible replay attack: "
-                    + messageIndexKey
+                    "Duplicate message index, possible replay attack: " +
+                    messageIndexKey
                 );
             }
             self._inboundGroupSessionMessageIndexes[messageIndexKey] = true;