1.1 --- a/imiptools/data.py	Fri Sep 18 23:45:56 2015 +0200
     1.2 +++ b/imiptools/data.py	Sat Sep 19 00:11:13 2015 +0200
     1.3 @@ -394,7 +394,7 @@
     1.4          previous RDATE properties and ignoring any RRULE properties.
     1.5          """
     1.6  
     1.7 -        old_values = set(self.get_date_values("RDATE"))
     1.8 +        old_values = set(self.get_date_values("RDATE") or [])
     1.9          new_rdates = []
    1.10  
    1.11          if self.has_key("RDATE"):
    1.12 @@ -407,7 +407,7 @@
    1.13          if new_rdates:
    1.14              self["RDATE"] = new_rdates
    1.15  
    1.16 -        return old_values != set(self.get_date_values("RDATE"))
    1.17 +        return old_values != set(self.get_date_values("RDATE") or [])
    1.18  
    1.19      def correct_object(self, tzid, permitted_values):
    1.20