Fix undefined method '/' error when the master file is still accessible

I couldn't think of a good test for this.  I'm reusing the offset variable which is setup earlier in the method by calling :to_i on options[:offset].  If options[:offset] is a string (which is should be) and the master file is still accessible by avalon then line 470 will attempt to divide the string by 1000.0 leading to a undefined method error.