Is.Empty constraint has unclear failure message

[GT]

Using v2.6 b2.

    var emptyEnumerable = GetEmptyIEnumerable();
    var emptyString = GetEmptyString();

    Assert.That(emptyEnumerable, Is.Empty);
    Assert.That(emptyString, Is.Empty);

If the methods under test in the above example return 'null' instead of the expected string or IEnumerable, the failure message states "System.ArgumentException: The actual value must be an IEnumerable".

At least the test is failing correctly, so the problem's not too bad, but it can be slightly non-intuitive to fix the test, especially when the actual value being tested is a string.

I'd expect it either to say "The actual value must be an IEnumerable or a string", or it to give a more precise answer, something like "The actual value was not empty" / "not empty but null".