diff --git a/pySim/transport/calypso.py b/pySim/transport/calypso.py
index b827d88..d7d9649 100644
--- a/pySim/transport/calypso.py
+++ b/pySim/transport/calypso.py
@@ -160,5 +160,5 @@
 
         return b2h(data), b2h(sw)
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "osmocon:%s" % (self._sock_path)