Field of merged message cannot be referenced in outer condition

[senier]

package Test is
   type Count is mod 2**32;
   type M1 is
      message
         F1 : Count;
      end message;
   type M2 is
      message
         F2 : M1
            then null if F2_F1 > 42;
      end message;
end Test;

$ rflx check test.rflx 
Parsing test.rflx
Processing Test
test.rflx:10:26: model: error: undefined variable "F2_F1" referenced