grml-prompt: right prompt can not be disabled (once it is enabled)

[jkirk]

% prompt -h grml
[...]
    The actual configuration is done via zsh's `zstyle' mechanism. The
    context, that is used while looking up styles is:

        ':prompt:grml:<left-or-right>:<subcontext>'

    Here <left-or-right> is either `left' or `right', signifying whether the
    style should affect the left or the right prompt. <subcontext> is either
    `setup' or 'items:<item>', where `<item>' is one of the available items.

    The styles:

        - use-rprompt (boolean): If `true' (the default), print a sad smiley
          in  if the last command a returned non-successful error code.
          (This in only valid if <left-or-right> is "right"; ignored otherwise)
[...]

But trying to do so does not disable the right prompt / sad smiley:

jkirk@executor ~ % /bin/false
1 jkirk@executor ~ % zstyle ':prompt:grml:right:setup' use-rprompt false           :(
jkirk@executor ~ % zstyle -T ':prompt:grml:right:setup' use-rprompt
1 jkirk@executor ~ %                                                              :(

The current workaround is this:

jkirk@executor ~ % zstyle ':prompt:grml:right:setup' items
jkirk@executor ~ % /bin/false
1 jkirk@executor ~ % 

(Putting zstyle ':prompt:grml:left:setup' use-rprompt false in ~/.zshrc.local works.)