=== modified file 'share/extensions/interp.py'
--- share/extensions/interp.py	2016-08-27 13:31:49 +0000
+++ share/extensions/interp.py	2017-01-07 15:01:25 +0000
@@ -110,8 +110,8 @@
 
     def tweenstyleunit(self, property, start, end, time): # moved here so we can call 'unittouu'
         scale = self.unittouu('1px')
-        sp = self.unittouu(start[property]) / scale
-        ep = self.unittouu(end[property]) / scale
+        sp = self.unittouu(start.get(property, '1px')) / scale
+        ep = self.unittouu(end.get(property, '1px')) / scale
         return str(sp + (time * (ep - sp)))
 
     def effect(self):