M-x customize-variable jack-set-rtlimits-program shows Hide jack-set-rtlimits-program: nil [ State ]: STANDARD. (mismatch) "mismatch" means the value nil, which is the default, is not allowed by its customize :type 'file. For a program name which can be nil for no such program one possibility is a choice style :type '(choice (const :tag "None" :value nil) string) as per for example socks-nslookup-program in socks.el.