问为什么会发生将数据类型varchar转换为bigint的错误？

EN

ALTER PROCEDURE [dbo].[Reports_UnDisbursedSalaryReport]
(
    @FromDate SMALLDATETIME,
    @ToDate SMALLDATETIME,
    @OfficeID INT = 3,
    @Type INT = 1,
    @EmployerUniqueID VARCHAR(15) = '',
    @EmployeeUniqueID VARCHAR(15) = ''
)
AS
BEGIN
    SET @EmployerUniqueID = 
        CASE 
           WHEN LEN(@EmployerUniqueID) <=0 
              THEN NULL 
              ELSE @EmployerUniqueID 
        END
    SET @EmployeeUniqueID = 
        CASE 
           WHEN LEN(@EmployeeUniqueID) <= 0
              THEN NULL 
              ELSE @EmployeeUniqueID 
        END

    DECLARE @Branches TABLE (BranchID INT)

    IF @Type = 1
    BEGIN
        INSERT INTO @Branches 
            SELECT BranchID 
            FROM vw_OrganizationTree 
            WHERE OrganizationID = @OfficeID
    END

    IF @Type = 2
    BEGIN
        INSERT INTO @Branches 
            SELECT BranchID 
            FROM vw_OrganizationTree 
            WHERE CompanyID = @OfficeID
    END

    IF @Type = 3
    BEGIN
        INSERT INTO @Branches 
            SELECT BranchID 
            FROM vw_OrganizationTree 
            WHERE BranchID = @OfficeID
    END

    SELECT 
        DATEADD(HOUR, 4, PF.EntryDateTime) 'EntryDateTime', 
        C.CompanyName, B.BranchName, 
        @FromDate as 'Fromdate', @ToDate as 'Todate', 
        E.EmployerName, RE.EmployeeName, RE.EmployeeUniqueID, 
        FE.IncomeFixedComponent, FE.IncomeVariableComponent,
        S.StatusDescription
    FROM
        File_EdrEntries FE 
    JOIN 
        RegisteredEmployees RE ON RE.EmployeeUniqueID = FE.EmployeeUniqueID
    JOIN 
        Employers E ON E.ID = RE.Employer_ID 
    JOIN 
        Branches B ON B.BranchID = E.Branch_ID 
    JOIN 
        companies C ON C.COMPANYID = B.COMPANY_ID
    JOIN 
        Statuses S ON S.StatusID = FE.Status_ID 
    JOIN 
        PAFFiles PF ON PF.ID = FE.PAFFile_ID
    WHERE 
        PF.EntryDateTime >= @FromDate 
        AND PF.EntryDateTime < DATEADD(DAY, 1, @ToDate) 
        AND Status_ID IN (1, 2) 
        AND E.Branch_id IN (SELECT BranchID From @Branches)
        AND CAST(E.EmployerID AS BIGINT) = ISNULL(CAST(@EmployerUniqueID AS BIGINT), CAST(E.EmployerID AS BIGINT))
        AND CAST(RE.EmployeeUniqueID AS BIGINT) = ISNULL(CAST(@EmployeeUniqueID AS BIGINT), CAST(RE.EmployeeUniqueID AS BIGINT))
END