Need help on this.

I'm just wondering why this error occurred. Below is the script concerned.

    SELECT loc.ID
    ,loc.LocCode
    ,loc.LocName
    ,st.StateName
    ,reg.RegionName
    ,ctry.CountryName
    ,ISNULL(CONVERT(DATE, loc.UpdatedDate), CONVERT(DATE,loc.CreatedDate)) AS [ModifiedDate]
    ,stf.Name AS [ModifiedBy]
FROM Spkr_Country AS ctry WITH (NOLOCK)
INNER JOIN Spkr_Location AS loc WITH (NOLOCK) ON ctry.ID = loc.CountryID
INNER JOIN Spkr_State AS st WITH (NOLOCK) ON loc.StateID = st.ID
INNER JOIN Spkr_Region AS reg WITH (NOLOCK) ON loc.RegionID = reg.ID
INNER JOIN Staff AS stf ON ISNULL(loc.UpdatedBy, loc.CreatedBy) = stf.StaffId
WHERE (loc.IsActive = 1)
    AND (
        (@LocCode = '')
        OR (
            @LocCode <> ''
            AND loc.LocCode LIKE @LocCode + '%'
            )
        )
    AND (
        (@RegionID < 1)
        OR (
            @RegionID > 0
            AND loc.RegionID = @RegionID
            )
        )
    AND (
        (@StateID < 1)
        OR (
            @StateID > 0
            AND loc.StateID = @StateID
            )
        )
    AND (
        (@CountryID < 1)
        OR (
            @CountryID > 0
            AND loc.CountryID = @CountryID
            )
        )

The error probably occurred here INNER JOIN Staff AS stf ON ISNULL(loc.UpdatedBy, loc.CreatedBy) = stf.StaffId The requirement that I wanted is that if the loc.UpdatedBy is null, it will use the loc.CreatedBy column. However, when I used this, it generated the error mentioned.

In the database, the loc.CreatedBy is not null while the loc.UpdatedBy is nullable.

I checked it by running the script but it's working fine.

How do I do with it? What's wrong with my code? Please help.