Concatenating sql table rows and converting a data type

Question

I'm concatenating multiple table rows into one row. Below code is the working only with nvarchar data type but not with int.

ALTER PROCEDURE [dbo].[spViewConcatenatedPolicyCInoForTaxInvoice_motorOnly]
    -- Add the parameters for the stored procedure here
    @TaxInvoiceNumber nvarchar(50)
AS
BEGIN

        DECLARE @MotorCINo int
        SET @MotorCINo=''

        SELECT @MotorCINo = @MotorCINo + MotorCINo +', '
        FROM    CashierSalesDetails
        WHERE TaxInvoiceNumber = @TaxInvoiceNumber
        SELECT  LEFT(@MotorCINo,LEN(@MotorCINo)-1) AS PolicyCINo

END

 below is the error:

Msg 245, Level 16, State 1, Procedure spViewConcatenatedPolicyCInoForTaxInvoice_motorOnly, Line 17
Conversion failed when converting the varchar value ', ' to data type int.

 Any is greatly appreciated.