diff --git a/pybis/src/python/pybis/pybis.py b/pybis/src/python/pybis/pybis.py
index e7bb1bd5a4ba80349bd26b510cf37289ab36181f..dab6d7eed9ee1e1ae89b857259ccab28fc60a115 100644
--- a/pybis/src/python/pybis/pybis.py
+++ b/pybis/src/python/pybis/pybis.py
@@ -969,7 +969,7 @@ class Openbis:
            self.mountpoint = None
 
 
-    def is_mounted(self, mountpoint, None):
+    def is_mounted(self, mountpoint=None):
         if mountpoint is None:
             mountpoint = getattr(self, 'mountpoint', None)