--- a/xpcom/idl-parser/typelib.py
+++ b/xpcom/idl-parser/typelib.py
@@ -154,17 +154,17 @@ def build_interface(iface, ifaces):
             return xpt.Param(type, in_=True, retval=True, dipper=True)
         return xpt.Param(type, in_=False, out=True, retval=True)
 
     def build_attr_param(a, getter=False, setter=False):
         if not (getter or setter):
             raise Exception("Attribute param must be for a getter or a setter!")
 
         type = get_type(a.realtype, getter and 'out' or 'in')
-        if set:
+        if setter:
             return xpt.Param(type)
         else:
             if isDipperType(type.tag):
                 # NB: The retval bit needs to be set here, contrary to what the
                 # xpt spec says.
                 return xpt.Param(type, in_=True, retval=True, dipper=True)
             return xpt.Param(type, in_=False, out=True, retval=True)