I don't know why I get this error.

"A number is required here!"

I receive this error on the following highlighted code:

if {tblConsignees.consignee} = {tblBatchDetail.destCustNumber} then
{tblBatchDetail.destCustNumber} + "(" + {tblConsignees.customerID} + ")"
else
{tblBatchDetail.destCustNumber} + "(Unk)"