READ-BDF/BDF-TO-FONTDESCRIPTOR fails on FontForge generated BDF of macOS Monaco font

[nbriggs]

Describe the bug
After reading the BDF for Monaco, calling BDF::BDF-TO-FONTDESCRIPTOR fails with a type-mismatch error, "(-1) is not a number" when (-1) was passed to IL:UNICODE-EXTEND-TRANSLATION, called from UNICODE-UNMAPPED, called from IL:UTOXCODE?

To Reproduce
Steps to reproduce the behavior:

Used FontForge to load macOS /System/Library/Fonts/Monaco.ttf and write it out, with default settings, creating amongst other sizes, Monaco-10.bdf

(SETQ MONACO10 (BDF:READ-BDF "{dsk}<~>Lisp>Monaco-10.bdf"))
(BDF:WRITE-BDF-TO-DISPLAYFONT-FILES MONACO10 "{dsk}<~>Lisp>" NIL NIL 'MONACO 10 'MRR 0 'DISPLAY)

Expected behavior
I expected it to produce a Monaco 10 display font file.

[nbriggs]

Monaco-10.bdf.gz