Explicit Value Error, Why?

Hello,

In the SP below, I get the following error when I run it:
Explicit value must be specified for identity column in table 'Tests' when IDENTITY_INSERT is set to ON.

From what I've read of the INSERT method, it says you can add a variable value. So I don;t understand why it won't take mine.

CREATE PROC qaspAddTestRecord AS

DECLARE @aKey bigint
DECLARE @aTestSuiteId bigint

exec qaspGetNewID 'Tests', @aKey OUT

SELECT @aTestSuiteId= NewID FROM ID WHERE TableName='TestSuite'

SET IDENTITY_INSERT Tests ON

INSERT INTO Tests (TestSuiteId, TestIDInternal) VALUES (@aTestSuiteId, @aKey)

SET IDENTITY_INSERT Tests OFF
GO


--PhB