I need some help with a MS SQL Query

Hello,

I am trying to create a stored procedure with the code below and I am getting the same error no matter what I do to the comparison of the MR.AgencyId, an int field and i.invUserLevel4 that is a varchar field and has some NULL values that need not to be compared.

Error:
Msg 245, Level 16, State 1, Line 3
Conversion failed when converting the varchar value 'NULL' to data type int.

Code:

use txu

SELECT
MR.AgencyId, AgencyName, FullName, PremiumsWrittenAmount, NewApps, SigPadCount,
CASE WHEN ISNULL(SUM(NewApps),0) = 0 THEN 0 ELSE CONVERT(int,(100 * (ISNULL(CONVERT(decimal(5,2), SUM(SigPadCount)), 0)/CONVERT(decimal(5,2), SUM(NewApps))))) END UsagePercentage,
COUNT(i.invSerialNo) as PadsOnHand
FROM
(
SELECT D.AgencyId, AgencyName, MarketingRepId, UAV.FullName
FROM txu.dbo.AgencyData AS D WITH (NOLOCK)
INNER JOIN txu.dbo.Agencies As A WITH (NOLOCK) ON A.AgencyDataId = D.AgencyDataId
INNER JOIN txu.dbo.UserAccountsView AS UAV WITH (NOLOCK) ON UAV.UserId = D.MarketingRepId
) AS MR
INNER JOIN fn_SigPadInfoByAgent('01/01/2011','01/10/2011') AS sp
      ON sp.AgencyId = MR.AgencyId
INNER JOIN NewSPDB.dbo.tblInventory AS i ON MR.AgencyId = convert(int,i.invUserLevel4)
where i.invUserLevel4 is not null
GROUP BY MR.AgencyId, AgencyName, FullName, PremiumsWrittenAmount, NewApps, SigPadCount