单据选择字段表达式出错

FLOT = F_PAEZ_Base.FName + F_PAEZ_Base1.FName + FDATE.ToString('yyyy-MM-dd')

FLOT.FName = F_PAEZ_Base.FName + F_PAEZ_Base1.FName + FDATE.ToString('yyyy-MM-dd')

不管我用上面的的哪个公式去实现转换，总是报下面的错误面的错误。

发生时间： 11:35:02
错误编号： PyExpressionEvaluator
错误信息： 表达式计算出错: FLOT.FName = F_PAEZ_Base.FName + F_PAEZ_Base1.FName + FDATE.ToString('yyyy-MM-dd') ( unsupported operand type(s) for +: 'str' and 'NoneType' )
===================================================
调用堆栈：
在 Kingdee.BOS.Scripting.PyExpressionEvaluator.Run(Expression expression, ExpressionContext context)
在 Kingdee.BOS.Business.DynamicForm.BusinessService.Calculate.DoAction(FormBusinessServiceArgs e)
在 Kingdee.BOS.Core.DynamicForm.FormBusinessServiceUtil.InvokeService(IDynamicFormView view, Object source, FormBusinessService service, String activeEntity, DynamicObject activeRow, Int32 rowIndex)
在 Kingdee.BOS.Core.DynamicForm.FormBusinessServiceUtil.InvokeServices(IDynamicFormView view, Object source, IEnumerable`1 services, Int32 actionType, String activeEntity, DynamicObject activeRow, Int32 rowIndex)
在 Kingdee.BOS.Web.DynamicForm.BusinessServiceCaller.InvokeFormBusiness(IDynamicFormView view, Object source, List`1 formBusinessServices, Int32 actionType, String entityKey, Int32 row)
在 Kingdee.BOS.Web.DynamicForm.BusinessServiceCaller.CallUpdateBusinessService(FieldAppearance fieldAppearance, Int32 row)
在 Kingdee.BOS.Web.DynamicForm.DynamicWebFormView.DoLookUpSetFieldValue(FieldAppearance ap, ILookUpField lookUpField, Int32 rowIndex, ListSelectedRow selectRow, String combinedFieldKey)
在 Kingdee.BOS.Web.DynamicForm.DynamicWebFormView.DoLookUpAfterShow(String key, Int32 row, ILookUpField lookUpField, FieldAppearance ap, FormResult result, String combinedFieldKey)
在 Kingdee.BOS.Web.DynamicForm.DynamicWebFormView.<>c__DisplayClass64.b__63(FormResult result)
在 Kingdee.BOS.Web.DynamicForm.DynamicWebFormView.<>c__DisplayClass43.b__41() 在 CallSite.Target(Closure , CallSite , Object , Object )
在 System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1)
在 Microsoft.Scripting.Interpreter.DynamicInstruction`3.Run(InterpretedFrame frame)
在 Microsoft.Scripting.Interpreter.Interpreter.Run(InterpretedFrame frame)
在 Microsoft.Scripting.Interpreter.LightLambda.Run2[T0,T1,TRet](T0 arg0, T1 arg1)
在 IronPython.Compiler.PythonScriptCode.RunWorker(CodeContext ctx)
在 IronPython.Compiler.RuntimeScriptCode.InvokeTarget(Scope scope)
在 Kingdee.BOS.Scripting.PyExpressionEvaluator.Run(Expression expression, ExpressionContext context)