Mule随机错误-无法绑定属性

我在表达式中随机得到这个错误。无法找到根本原因

此处payload.SupplierId始终为空。我用一个流场变量值来设置它

异常堆栈是：

1. object is not an instance of declaring class (java.lang.IllegalArgumentException)
    sun.reflect.NativeMethodAccessorImpl:-2 (null)
2. unable to bind property (java.lang.RuntimeException)
    org.mvel2.optimizers.impl.refl.nodes.SetterAccessor:43 (null)
3. Execution of the expression "payload.SupplierId = flowVars.SupplierRequestData.SupplierId" failed. (org.mule.api.expression.ExpressionRuntimeException)
    org.mule.el.mvel.MVELExpressionLanguage:213 (http://www.mulesoft.org/docs/site/current3/apidocs/org/mule/api/expression/ExpressionRuntimeException.html)
4. Execution of the expression "payload.SupplierId = flowVars.SupplierRequestData.SupplierId" failed. (org.mule.api.expression.ExpressionRuntimeException). Message payload is of type: SupplierResponse (org.mule.api.MessagingException)
    org.mule.execution.ExceptionToMessagingExceptionExecutionInterceptor:35 (http://www.mulesoft.org/docs/site/current3/apidocs/org/mule/api/MessagingException.html)

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SupplierRequestData是一个POJO。我想知道这是否是访问流变量内字段的正确方法？或者它应该是flowVars['SupplierRequestData']。SupplierId如果不共享代码，您如何期望解决方案？请分享您的代码，以便很容易发现问题