libsmi error char-illegal-string

Error text: illegal character `__' (0x__) in quoted string

Reference: RFC 2578 section 3.1.1

   Some clauses in a macro invocation may take a character string as a
   textual value (e.g., the DESCRIPTION clause).  Other clauses take
   binary or hexadecimal strings (in any position where a non-negative
   number is allowed). 
    
   A character string is preceded and followed by the quote character
   ("), and consists of an arbitrary number (possibly zero) of:
    
      - any 7-bit displayable ASCII characters except quote ("),
      - tab characters,
      - spaces, and 
      - line terminator characters (\n or \r\n).
    
   The value of a character string is interpreted as ASCII.