diff --git a/ohmpi.py b/ohmpi.py
index 82ca0b477d6429a0d980c989fb620ce096064a92..a72513a2104a6b41f2a0af2d25f3737cd266fc76 100644
--- a/ohmpi.py
+++ b/ohmpi.py
@@ -1153,7 +1153,7 @@ class OhmPi(object):
         """
         self.reset_mux() # All mux boards should be reset even if we only want to test one otherwise we might create a shortcut
         if mux_id is None:
-            self._hw.test_mux(activation_time)
+            self._hw.test_mux(activation_time=activation_time)
         else:
             self._hw.mux_boards[mux_id].test(activation_time=activation_time)