As the bookmark is from a form field, the error relates to the fact that the field construction is toggled. It would be preferable to leave the field locked and use

Personally I would give thought to changing the form fields to content controls as processing such a form is much faster with content controls, especially in Word 2010. You can convert them with http://www.gmayor.com/ExtractDataFromForms.htm

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com