Transact SQL :: Converting Data Type Nvarchar To Bigint

This is a common error for SQL Server, but I got it in a uncommon way.I have a table called - tblIDNumber where there are two columns - IDN_Number [NVarchar(200)] and Temp [BigInt]

If I run,
SELECT  *
FROM    dbo.tblIDNumber
WHERE   IDN_IDNumberTypeStaticValue = 33
        AND IDN_Removed = 0
        AND CAST(IDN_Number AS BIGINT) = 1

SQL Server give me the error:
Msg 8114, Level 16, State 5, Line 1
Error converting data type nvarchar to bigint.

I first thought IDN_Number in type 33 has characters, but it doesn't, becasue the below query works!!!

UPDATE dbo.tblIDNumber
SET Temp = CAST(IDN_Number AS BIGINT)
WHERE  IDN_IDNumberTypeStaticValue = 33
        AND IDN_Removed = 0

To workaround, I ran the query,

UPDATE dbo.tblIDNumber
SET IDN_Number = '123'
WHERE  IDN_IDNumberTypeStaticValue = 33
        AND IDN_Removed = 0

and then I ran the first query, and SQL Server does NOT give me the same error - Msg 8114, Level 16, State 5, Line 1 Error converting data type nvarchar to bigint.

Second query approved there is nothing wrong from converting the value in IDN_Number to a BigInt, but the third query gave the hint that data might be the cause?????

finally, I found the root cause to be an index that the first query uses :

CREATE NONCLUSTERED INDEX [IX_tblIDNumber_Covering] ON [dbo].[tblIDNumber]
(
[IDN_Removed] ASC,
[IDNumberCode] ASC
)
INCLUDE ( [IDN_Number],
[IDN_Reference]) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 85) ON [PRIMARY]
GO

If I remove the index, the first query works without the error.