strange length check error

[Mehdi Saada <00120260a@gmail.com>]

This : Put(Ch_Bounded1 & ' ' );
Calls on that:
function "&"
     (Left  : in     Bounded_String;
      Right : in     Character)
      return Bounded_String
   is
   begin
      if LENGTH(LEFT) + 1 > MAX_LENGTH then
         raise LENGTH_ERROR with "ERROR '&': LEFT.LENGTH := " & NATURAL'IMAGE(LENGTH(LEFT)) & " + 1 > MAX_LENGTH";
      else
 @@@     return (LENGTH(LEFT) + 1, LEFT.DATA(1..LENGTH(LEFT)) & RIGHT);
      end if;
   end "&";

The line with @@@ raises this:
raised CONSTRAINT_ERROR : p_bounded_g-p_new_bounded_g.adb:34 length check failed
The value of CH_BOUNDED1 at that moment is "pour tester le debut", its length is 21, and MAX_LENGTH est 80.