What it usually means is that Word has got confused about the type of data
in a column in your data source, and builds the wrong piece of query
language. For example, if it thinks the field is a number, it should build a
bit of language like "WHERE myfield = 123", but if it thinks the field is a
piece of text it should use "WHERE myfeild = '123'). This can occur because
the data converters that get data on behalf of Word may logically have to
make guesses about what data is in a column.
Although you may not always be able to solve this problem easily, (we'd at
least need to know which version of Word and what the data source was to
start thinking about a solution), if your data source is Excel and you have
a column where some of the data is numeric and some is alphanumeric, you may
need to refomat the column in Excel as text and reconnect Word to your data
source.