diff --git a/pySim/transport/serial.py b/pySim/transport/serial.py
index f4b1621..e5f7bdb 100644
--- a/pySim/transport/serial.py
+++ b/pySim/transport/serial.py
@@ -237,5 +237,5 @@
         # Return value
         return b2h(data), b2h(sw)
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "serial:%s" % (self._sl.name)