Ron, to expand on what Jeff points out, LastDoc$ is a variable returned from the Crystal Reports engine after the print job is complete to tell the program the last document that was processed by Crystal so that the records can be flagged as printed. In the case of P/R checks, the LastDoc$ should be set to the check number from the PR_31 file, but it's apparently coming back as short, probably null.
Go to the form (PRWFXA1.RPT) and make sure that there's a formula named ""LastDocument"" with a definition of """". When the program fires off the Crystal Report, it will fill in that formula value with {PR_31CrystalCheckHeader.CheckNumber}.