I put a thinrule in a table as follows:

\starttext
\bTABLE
    \bTR\bTD\thinrule\eTD\eTR
\eTABLE
\stoptext

It worked fine under MKII.  However, when I switched to MKIV (both current and 
beta) I got the following error message:

! Missing { inserted.
<inserted text>
                {
<to be read again>
                   &
<inserted text> ...blrow \makeTBL 1 1 \nexttblcol
                                                  \endtblrow
\flushtbltoks ->\begintbl \the \tbltoks
                                        \endtbl
\endTBL ...x {\trialtypesettingtrue \flushtbltoks
                                                  }\lettbldis \maximumcol 
\z...
\eTABLE ...}}\eTBL }\removeunwantedspaces \endTBL
                                                  \stopTBLprocessing \fi \eg...
l.4 \eTABLE


Is this a bug?
Is there a workaround?

Thanks,
Bart