`This` reference not accessible when overloading an operator?

[Enjoys Math via Digitalmars-d-learn]

I have an Integer class in integer.d.  A RationalNumber class in 
rational_number.d, and they each import each other (so that could 
be the issue).  However, this is not working:

   Symbol opBinary(string op : "/")(const Integer z) const
   {
      return new RationalNumber(this, z);
   }

Getting:

Error: class `rational_number.RationalNumber` member `this` is 
not accessible		
Error: template instance `integer.Integer.opBinary!"/"` error 
instantiating