diff --git a/src/crypto/algorithms/megolm.js b/src/crypto/algorithms/megolm.js
index 57f6bab32..1aaa67af6 100644
--- a/src/crypto/algorithms/megolm.js
+++ b/src/crypto/algorithms/megolm.js
@@ -641,8 +641,9 @@ MegolmDecryption.prototype.decryptEvent = async function(event) {
         if (e.message === 'OLM.UNKNOWN_MESSAGE_INDEX') {
             this._requestKeysForEvent(event);
         }
+        // TODO: make OlmDevice throw base.DecryptionErrors too
         throw new base.DecryptionError(
-            "OLM_UNKNOWN_MESSAGE_INDEX",
+            "OLM_DECRYPT_GROUP_MESSAGE_ERROR",
             e.toString(), {
                 session: content.sender_key + '|' + content.session_id,
             },