diff --git a/lib/encryption/encryption.dart b/lib/encryption/encryption.dart
index 8c9ee7b..db56cb5 100644
--- a/lib/encryption/encryption.dart
+++ b/lib/encryption/encryption.dart
@@ -160,24 +160,18 @@ class Encryption {
       final decryptResult = inboundGroupSession.inboundGroupSession
           .decrypt(event.content['ciphertext']);
       canRequestSession = false;
-      final messageIndexKey = event.eventId +
+      // we can't have the key be an int, else json-serializing will fail, thus we need it to be a string
+      final messageIndexKey = 'key-' + decryptResult.message_index.toString();
+      final messageIndexValue = event.eventId +
+          '|' +
           event.originServerTs.millisecondsSinceEpoch.toString();
       var haveIndex = inboundGroupSession.indexes.containsKey(messageIndexKey);
       if (haveIndex &&
-          inboundGroupSession.indexes[messageIndexKey] !=
-              decryptResult.message_index) {
+          inboundGroupSession.indexes[messageIndexKey] != messageIndexValue) {
         // TODO: maybe clear outbound session, if it is ours
         throw (DecryptError.CHANNEL_CORRUPTED);
       }
-      final existingIndex = inboundGroupSession.indexes.entries.firstWhere(
-          (e) => e.value == decryptResult.message_index,
-          orElse: () => null);
-      if (existingIndex != null && existingIndex.key != messageIndexKey) {
-        // TODO: maybe clear outbound session, if it is ours
-        throw (DecryptError.CHANNEL_CORRUPTED);
-      }
-      inboundGroupSession.indexes[messageIndexKey] =
-          decryptResult.message_index;
+      inboundGroupSession.indexes[messageIndexKey] = messageIndexValue;
       if (!haveIndex) {
         // now we persist the udpated indexes into the database.
         // the entry should always exist. In the case it doesn't, the following
diff --git a/lib/encryption/utils/session_key.dart b/lib/encryption/utils/session_key.dart
index b54ca32..471c1f4 100644
--- a/lib/encryption/utils/session_key.dart
+++ b/lib/encryption/utils/session_key.dart
@@ -26,7 +26,7 @@ import '../../src/utils/logs.dart';
 
 class SessionKey {
   Map<String, dynamic> content;
-  Map<String, int> indexes;
+  Map<String, String> indexes;
   olm.InboundGroupSession inboundGroupSession;
   final String key;
   List<String> get forwardingCurve25519KeyChain =>
@@ -60,9 +60,14 @@ class SessionKey {
         Event.getMapFromPayload(dbEntry.senderClaimedKeys);
     content =
         parsedContent != null ? Map<String, dynamic>.from(parsedContent) : null;
-    indexes = parsedIndexes != null
-        ? Map<String, int>.from(parsedIndexes)
-        : <String, int>{};
+    // we need to try...catch as the map used to be <String, int> and that will throw an error.
+    try {
+      indexes = parsedIndexes != null
+          ? Map<String, String>.from(parsedIndexes)
+          : <String, String>{};
+    } catch (e) {
+      indexes = <String, String>{};
+    }
     roomId = dbEntry.roomId;
     sessionId = dbEntry.sessionId;
     _setSenderKey(dbEntry.senderKey);