I am facing a strange problem in creating trigger in MS SQL. I have a query that executes with out an error but when I place it within trigger body, it gives error

Invalid column name 'ScreenName'.

I am placing whole trigger code here. Please trace the issue.

CREATE TRIGGER [dbo].[tr_tbFieldLabels_FieldLabelLength] ON [dbo].[tbFieldLabels]
AFTER INSERT
AS
    Update tbFieldLabels 
    Set TextBoxLength = (SELECT top 1 TextboxLength 
                              FROM  tbFieldLabelsSource FLS
                              WHERE FLS.ScreenName = Inserted.ScreenName
                              AND  tbFieldLabelsSource.SystemName = Inserted.SystemName)
     FROM tbFieldLabels , Inserted WHERE tbFieldLabels.ID = Inserted.ID
GO

Please help me ASAP.