9 digit zip code imports as 0, not 9 digits
I am using MS Word and MS Excel 2003. From MS Word, I have done a MAIL MERGE, and some of the zip codes are 5 digit, while others are 9 digit, as in 12345-6789. After doing the mail merge, and previewing the letters, the 9 digit zip codes get imported as 0. Certainly the MS Word software should be able to recognize a 9 digit zip code as a zip code? The 5 digit zip codes import properly as 5 digit zip codes.
How can I make the 9 digit zip codes display properly after doing the mail merge into MS Word? What am I doing wrong?