🚀 go-pugleaf

When parsing an SGML file in FrameMaker 5.5 + SGML, the SGML Parser
generates the following error:

"Out-of-Context data ended document element (TREATYDOCTYPE) and parse.
C:\Documents\SGML\text.sgm; line 144"

The online help indicates that: 
>>>>>"...there is a markup, declaration subset, or DTD error. A
non-SGML character, as defined by the concrete syntax, was found in
markup. ....the character syntax for a character reference is:

&#function-name;
or
&#character-number; "<<<<<

I tested by moving the character reference from one line to another in
the SGML file referenced above and that solved the problem!!! I went
further and created 2 references:
A. one character reference on line 100;
B. and another character reference on line 144. 
I run the SGML parser and had the same error. I then moved reference A
to line 144 and reference B to line 100 and to my surprise the
document parsing ended with NO errors!!!

Would someone help me to understand what could be happening in this
situation?

Thank you, sousa.