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user_id=6405

The nosy reactor has some code that checks for changes to 
assignedto and modifies the nosy list accordingly. You need to change 
line 144 from: 
 
if newvalues.has_key('assignedto'): 
 
to 
 
if newvalues.has_key('assignedto') and newvalues['assignedto'] is not 
None: