preconditions and interfaces

[Antonio Corbi via Digitalmars-d-learn]

Hi all,

Playing with interfaces and preconditions in methods I get 
strange results with dmd-2.0.84.0 but also with dmd-nightly. My 
code is like this:
-------------
import std.stdio;
interface Thing2D {
  void width(int w)
    in {
      writeln("Thing2D.width contract w = ",w);
      assert(w > 0);
    }
}

class Line : Thing2D {
  override void width(int w)
    in {
      writeln("Line.width contract w = ",w);
      assert(w >= 0);
    }
  do {
    writeln("Line.width: w = ", w);
  }
}

void main() {
  auto l = new Line;
  l.width(-1);
}

---------------

1) With dmd-2.084.0 I get:
./ifaceprecond
Thing2D.width contract w = 2
Line.width: w = -1

2) With dmd-nightly (as of today) I get random values for 
interface's 'w':
./ifaceprecond
Thing2D.width contract w = 647271536
Line.width: w = -1

I think this should be an error caught by the contract, isn't it?

Thx!
Antonio