diff --git a/pykat/detectors.py b/pykat/detectors.py
index a99ca3df72f05469926a9e44f049db6aa31e74d6..f66dc9510e92ec0df335d3e4c51488c78b01ccfb 100644
--- a/pykat/detectors.py
+++ b/pykat/detectors.py
@@ -97,7 +97,7 @@ class photodiode(Detector):
     def phi(self): return self.__phi
 
         
-    def __init__(self, name, node, senstype=None, num_demods=0, demods=[]):        
+    def __init__(self, name, node, senstype="", num_demods=0, demods=[]):        
         Detector.__init__(self, name, node)
         if num_demods>2:
             raise NotImplementedError("pd with more than two demodulations not implemented yet")