The problem is that you are using two different conditions – one in the test, and the other in the constructor.

The condition in the constructor is implied, but still there. The constructor you are using is the same as new Uri( myUrl1, UriKind.Absolute ) and fails because the Url is not absolute.

If you change the constructor to match the test: new Url( myUrl1, UriKind.RelativeOrAbsolute ), then the constructor will succeed, as well.

Alternatively, changing the flag in the test to use UriKind.Absolute will cause the test to fail, matching the response of the constructor.