A similar question came up a cople of days ago...

A few questions:
a. Which version of Word?
b. What is the data source?
c. If the data source is Excel, does the Excel column containing the codes
have a mixture of code types
(e.g. numeric US codes, alphanumeric Canadian codes, other?). If so, are the
first few codes in the column all numeric? Or is the column formatted as
numeric?

If the column is formatted as numeric or the first few codes are all-numeric
codes, Word may treat the entire column as numeric.If that is the problem
you
may be able to fix it by changing the column format (make a copy of your
spreadsheet first!) so that it is text, or changing the way Word connects to
Excel (I'd try checking Word Tools|Options|General|"confirm conversions at
open", going through the connection process again, and selecting the DDE
option when it is offered). If you change the column format, you will
probably still need to rconnect to the data to get Word to "see" the change.

--
Peter Jamieson - Word MVP