Crystal Script条件语句'此字段名称未知'错误

Question

这是我目前的条件声明

IF {AR_DEBIT.InvoiceNo} = "" THEN {AR_DEBIT.Description}
ELSE {AR_DEBIT.Description} & "(" & {AR_DEBIT.InvoiceNo} & " - " & ToText({AR_DEBIT.InvoiceDate}, "dd-MMM-yyyy") & ")"

不工作...提示错误IF {AR_DEBIT.InvoiceNo} = "" THEN this field is not known bla bla like that that ...

当我删除条件时，

{AR_DEBIT.Description} & "(" & {AR_DEBIT.InvoiceNo} & " - " & ToText({AR_DEBIT.InvoiceDate}, "dd-MMM-yyyy") & ")"

报告显示没有错误......任何人？