diff --git a/matrix/encryption.py b/matrix/encryption.py
index 28e684a..f2c826b 100644
--- a/matrix/encryption.py
+++ b/matrix/encryption.py
@@ -222,11 +222,14 @@ class Olm():
                         continue
 
                 plaintext = session.decrypt(message)
-                break
+                return plaintext
             except OlmSessionError:
                 pass
 
-        session = self._create_session(sender, sender_key, message)
+        try:
+            session = self._create_session(sender, sender_key, message)
+        except OlmSessionError:
+            return None
 
         try:
             plaintext = session.decrypt(message)
diff --git a/matrix/events.py b/matrix/events.py
index e3127e2..4af350a 100644
--- a/matrix/events.py
+++ b/matrix/events.py
@@ -475,6 +475,9 @@ class MatrixSyncEvent(MatrixEvent):
         olm = server.olm
         plaintext = olm.decrypt(sender, sender_key, message)
 
+        if not plaintext:
+            return None
+
         # TODO check sender key
         parsed_plaintext = json.loads(plaintext, encoding='utf-8')
         decrypted_sender = parsed_plaintext["sender"]