Use an If...then...Else... field
{ IF { MERGEFIELD fieldname } <> "" { MERGEFIELD fieldname }
"__________________" }
You must use Ctrl+F9 to insert each pair of field delimiters { }

Signature
Hope this helps.
Please reply to the newsgroup unless you wish to avail yourself of my
services on a paid consulting basis.
Doug Robbins - Word MVP
>I am typing up a merge document where not every field has a corresponding
> entry. How can I create a default where these fields are filled in with a
> blank line?