The error is most likely because you used typed braces instead of field braces around 'NUMPAGES'.

An alternative approach is to insert a standard 'page x of y' numbering scheme into your footer, then select the 'y' part and press Ctrl-F9 to enclose that in a new field, thus {50}, then type '=' before the '50' and '-4' after it, so you end up with {=50-4}, then press F9 to update.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]