用户在应用促销代码时收到此错误，我进行了很多研究，但找不到此错误的原因。
环境是 IBM Websphere Commerce 7、DB2。
com.ibm.commerce.order.calculation.FinalizeDiscountCalculationUsageCmdImpl performExecute CMN3201E:
An unknown error occurred while the Rules System was processing..
com.ibm.commerce.marketing.promotion.runtime.PromotionRuntimeException:
com.ibm.commerce.exception.ECApplicationException:
A generic promotion system exception occured. The exception is as follows:
"The XPath query expression "/PX_CDUSAGE[CODE_FOR_REFERENCE=]" is invalid."
at com.ibm.commerce.marketing.promotion.runtime.PromotionArgument.finalizePromotionArgument(PromotionArgument.java:868)
at com.ibm.commerce.order.calculation.FinalizeDiscountCalculationUsageCmdImpl.performExecute(FinalizeDiscountCalculationUsageCmdImpl.java:104)
at com.ibm.commerce.command.ECCommandTarget.executeCommand(ECCommandTarget.java:157)
at com.ibm.ws.cache.command.CommandCache.executeCommand(CommandCache.java:332)
自 2 天以来，我一直试图找出此错误的根本原因，但找不到任何东西。任何帮助将不胜感激。