🚀 go-pugleaf

For some time, I have been offerring the code below to people who wish
to have uppercase work with characters like \aa.  It had worked well and
it seemed like an obvious fix.  I had never tested it with \mark before.
%%%%%%%%%%%% file starts here %%%%%%%%%%%%%%

\let\uc=\uppercase
\newif\ifucase
\def\ss{\ifucase SS\else\char"19\fi}
\def\aa{\ifucase\AA\else\accent23a\fi}
\def\ae{\ifucase\AE\else\char"1A\fi}
\def\oe{\ifucase\OE\else\char"1B\fi}
\def\o{\ifucase\O\else\char"1C\fi}
\def\uppercase#1{{\ucasetrue \uc{#1}}}

\mark{\uppercase{chapter}}
\bye
%%%%%%%%%%%% file ends here %%%%%%%%%%%%%%

But the latest recipient of this code wrote me that it caused an error
message when he used \chapter.  After some detective work, I isolated
the problem to a bad interaction with \mark.  Here a sample file.  The
error message is

! Incomplete \iffalse; all text was ignored after line 11.
<inserted text>
                \fi
<to be read again>
                   \bye
l.12 \bye

Notice that the \ifucase is executed only when one of the special
characters is executed and this does not happen in the example.  Only
the \ucasetrue is actually executed.  Anyone have any ideas why this is
happening?  Or what to do about it?

BTW, the even simpler file
\newif\iftest
{\testtrue}
\bye
will also elicit the error.

Michael Barr