Таки читайте RFC 1178:

      Don't use digits at the beginning of the name.

         Many programs accept a numerical internet address as well as a
         name.  Unfortunately, some programs do not correctly
         distinguish between the two and may be fooled, for example, by
         a string beginning with a decimal digit.

         Names consisting entirely of hexadecimal digits, such as
         "beef", are also problematic, since they can be interpreted
         entirely as hexadecimal numbers as well as alphabetic strings.

(Ответ для Michael Shigorin на комментарий #1)
> Таки читайте RFC 1178:
> 
>       Don't use digits at the beginning of the name.
> 
>          Many programs accept a numerical internet address as well as a
>          name.  Unfortunately, some programs do not correctly
>          distinguish between the two and may be fooled, for example, by
>          a string beginning with a decimal digit.
> 
>          Names consisting entirely of hexadecimal digits, such as
>          "beef", are also problematic, since they can be interpreted
>          entirely as hexadecimal numbers as well as alphabetic strings.

Таки надо откорректировать текст ошибки, так как если считать фичей описанное в данном багрепорте поведение, то текст ошибки должен корреткно оповещать пользователя об том, сообщая, что первым символом не может быть цифра потому-что...