The first in a pair of character numbers in the SWITCHES part of the SGML Declaration must not be a Digit, UC Letter or LC Letter, and it must be a markup character.

#0058   Markup Error

ISO 8879, 13.4.1

The first in a pair of character numbers in the SWITCHES part of the SGML Declaration must not be a Digit, UC Letter or LC Letter, and it must be a markup character.