System.Web.HttpException: 'http://localhost:3434' is not a valid virtual path..

Question

I am writing unit test cases using mstest. one of the method will change the Http to Https.

string url=http://localhost:3434
string expected=https://localhost:3434
string actual=ReplcaeTheUrl(url);//this will replace Http to Https
Assert.AreEqual(expected,actual);

Here my test case are failing by giving an exception

 System.Web.HttpException: 'http://localhost:3434' is not a valid virtual path..

What could be the issue?