Why does this IF statement take longer that others?

I have a unit test with different test cases (ie exactly the same code gets run but with different parameters).

One 3 of the test cases take around 26ms but one consistently takes around 500ms

the following is the if statement which gets run

public Boolean Test1()
{
  bool invalid = false;

  if (!string.IsNullOrEmpty(String1)&&!string.IsNullOrEmpty(String2))
  {
    invalid = true;
  }

  return invalid;
}

Now the case that takes the long time is setup with string1 being set to a valid string and string2 being set to null.

The other cases are string1=null,string2=null and string1=null, string2=valid string and string1=valid string, string2=valid string.

Can anyone see why the one arrangement would take almost 20 times longer than the others?