diff --git a/slixmpp_omemo/__init__.py b/slixmpp_omemo/__init__.py
index 5a9f553..f7320cb 100644
--- a/slixmpp_omemo/__init__.py
+++ b/slixmpp_omemo/__init__.py
@@ -659,7 +659,8 @@ class XEP_0384(BasePlugin):
 
         return await self._omemo().getTrustForJID(jid.bare)
 
-    def is_encrypted(self, msg: Message) -> bool:
+    @classmethod
+    def is_encrypted(cls, msg: Message) -> bool:
         return msg.xml.find('{%s}encrypted' % OMEMO_BASE_NS) is not None
 
     async def decrypt_message(