This is the problem. In IE6, 'toString' will not be enumerated, even
if it is an explicit property in an Object:
lzx> for (var k in {foo: 1, toSTring: 2}) Debug.write(k)
foo
toSTring
lzx> for (var k in {foo: 1, toString: 2}) Debug.write(k)
foo
lzx>