Malformed LyX document or bug?

[Scott Kostyshak]

I was sent the attached document. It opens without error in LyX 2.3.x,
but I get "Document header error" when opening in master along with the
following terminal messages:

  Warning: Malformed LyX file: Missing '\textclass'.
  Warning: Malformed LyX document: Missing \font_osf.
  Warning: Malformed LyX document: Missing \font_roman.
  Warning: Malformed LyX document: Missing \font_sans.
  Warning: Malformed LyX document: Missing \font_typewriter.
  Warning: Malformed LyX document: Missing \font_roman.
  Warning: Malformed LyX document: Missing \font_sans.
  Warning: Malformed LyX document: Missing \font_typewriter.
  Warning: Malformed LyX document! Missing \papersize header.
  Warning: Malformed LyX document! Missing \output_changes header.
  Warning: Malformed LyX document! Missing \maintain_unincluded_children.
  BufferParams::readToken(): Unknown token: \begin_document

Is it a valid .lyx file?

Scott
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