diff --git a/src/calibre/devices/mtp/test.py b/src/calibre/devices/mtp/test.py
index c273bac5e0..4eaf28a385 100644
--- a/src/calibre/devices/mtp/test.py
+++ b/src/calibre/devices/mtp/test.py
@@ -239,10 +239,12 @@ class TestDeviceInteraction(unittest.TestCase):
 
         # Test get_filesystem
         used_by_one = self.measure_memory_usage(1,
-                self.dev.dev.get_filesystem, self.storage.object_id)
+                self.dev.dev.get_filesystem, self.storage.object_id, lambda x:
+                x)
 
         used_by_many = self.measure_memory_usage(5,
-                self.dev.dev.get_filesystem, self.storage.object_id)
+                self.dev.dev.get_filesystem, self.storage.object_id, lambda x:
+                x)
 
         self.check_memory(used_by_one, used_by_many,
                 'Memory consumption during get_filesystem')