Package: gnus
Version: 5.110018

I get the following backtrace when trying to send:

Debugger entered--Lisp error: (scan-error "Containing expression ends prematurely" 1107 1108)
  scan-sexps(1107 1)
  forward-sexp(1)
  mml-read-tag()
  mml-parse-1()
  mml-parse()
  mml-generate-mime()
  message-encode-message-body()
  message-send-mail(nil)
  message-send-via-mail(nil)
  message-send(nil)
  message-send-and-exit(nil)
  call-interactively(message-send-and-exit nil nil)

Upon investigation, it looks like something is removing a space