AssertExpression states that a message if given must be convertible to char[].  That would prevent the use of a string constant in AssertExpression, since it is (immutable)char[] and not implicitly convertible to char[].

Here's the existing text:

The second Expression, if present, must be implicitly convertible to type char[]. It is evaluated if the result is false, and the string result is appended to the AssertError's message.