Object reference not set to an instance of an object...sometimes?

I have a application in VB.NET 2003 using Crystal Reports to create reports from an Access database. The problem I am running with one of my customers is that on two of the computers, when running crystalreportviewer for several reports, it returns the message in my subject line. There is one computer that is able to run all reports without a problem. They are all Windows 98 PCs. The reports also view correctly on my Windows 98 PC I use for testing. I do have the convertnullfieldtodefault set to true. Any ideas?

Thanks