Error parsing θ character

[oneingan]

This excerpt:

(prot-emacs-elpa-package 'altcaps
  ;; Force letter casing for certain characters (for legibility).
  (setq altcaps-force-character-casing
        '((?i . downcase)
          (?l . upcase)
          (?θ . downcase))))

fail with error error: Unrecognized token on line 6: ?θ . downcase))

I have unicode emojis in other parsed files doing well.