[rules-users] String.format question in the RHS

Hello, Can anyone see an issue with the following RHS? The String.format is failing. I've copied it straight into java to verify my intentions and it works there. What I'm trying to accomplish is to concatenate the string UH005 with a sequence number that is a fixed 10 digits, zero filled. So if CurrentClaimSeq_48 is the number 123, then I expect that TRN02_Check... would be populated with UH0050000000123. Again, this is working fine in straight java, but not in a rule. What am I missing? Is there a better way to try to accomplish this in a rule? rule "L1000_TRN02_UHC_BPA" dialect "mvel" when CLM_835_SUPP( seq : currentClaimSeq_48 != ( null ) ) CLM_DET_LOAD( ) trn : x221TransactionHeader_TRN_ReassociationTraceNumber( ) then trn.setTRN02_CheckOrEftTraceNumber( "UH005" + String.format ("%010d", seq) ); end Here are the data types of the two fields involved. private Integer CurrentClaimSeq_48; public String TRN02_CheckOrEftTraceNumber; Thank you! Dean