[NTG-context] meaning of error message?

I have been experimenting with tagging. But my attempts with two files now have generated this sort of error message: lua error > lua error on line 62 in file c_Int-A002_Acerbi.tex: .../ConTeXt/tex/texmf-context/tex/context/base/strc-tag.lua:407: bad argument #2 to 'lpegmatch' (string expected, got boolean) stack traceback: [C]: in function 'lpegmatch' .../ConTeXt/tex/texmf-context/tex/context/base/strc-tag.lua:407: in function 'strippedtag' .../ConTeXt/tex/texmf-context/tex/context/base/back-exp.lua:740: in function <.../ConTeXt/tex/texmf-context/tex/context/base/back-exp.lua:739> (...tail calls...) Could someone tell me what this means—is there are error in my encoding or a problem in lua? Alan