Using A SP To Merge Data

I am using the SP below to try to copy some data from one table to another and I am getting an error. Does anyone have any idea what would be causing it.

Alter Procedure usp_ImportData

As

truncate table tblLineItems

set identity_insert tblLineItems on

INSERT INTO
tblLineItems (ReportPreparer, ReportDate, REV, CalDate, Limits, RMAReport, FailureCause, DispositionofProduct, ReturntoStock, Trash)
SELECT
a.[Report Preparer], a.[Report Date], a.[REV], a.[Cal Date], a.[Limits], a.[RMA Report], a.[Failure Cause], a.[Disposition of Product], a.[Return to Stock], a.[Trash]

FROM
tblRMA a

INNER JOIN
tblLineItems b ON (a.[RMA Number]= b.RMANumber)

set identity_insert tblLineItems off


The error I am getting is "Explicit Value must be specified for identity column in table 'tblLineItems' when IDENTITY_INSERT is set to ON"