Standard value of defcustom doesn't match with type

To reproduce:

emacs -Q
(load-theme 'modus-operandi 'no-confirm 'no-enable)
M-x customize-option RET modus-themes-headings

Custom reports a mismatch, because the standard value: ((t . nil)) doesn't match the defcustom type.

More specifically, nil is not part of the choices allowed.