Did you change the Zip field to text. It may only contain numbers in the US but it is a "code" NOT a number you do arithmetic with so like phone numbers and SSN and license numbers, should always be stored as a string.
Living in New England where our Zips start with leading zeros, I run into this mistake all the time. Many times the problem is caused by Excel being the intermediary. If you use Excel, you have to coerce Excel into storing the zip as a string.