Calling stored procedure from another stored procedure in sqlserver

Question

Hi,

 

I have the following stored procedure in a Sql Server Database

CREATE PROCEDURE [dbo].[UP_SELECT_PO_NUMBER]
                                    @StoreBranchID int,
                                    @mPurchaseOrderNumber NVARCHAR(13) OUTPUT
                                    AS
--DECLARE @StoreBranchID int
--SELECT @StoreBranchID = 4
--DECLARE @mPurchaseOrderNumber nvarchar(13)

DECLARE @lpo int
DECLARE @mPO int
SET @mPurchaseOrderNumber=''
SELECT
        @mPO= cast(right(RTRIM(PurchaseOrderNumber),5) as int) + 1
FROM
        PurchaseOrder
WHERE
        PurchaseOrderID = (SELECT
                                    Max(purchaseOrderID)
                           FROM
                                    purchaseorder
                           WHERE
                                    storebranchID = @StoreBranchID)
SET @lpo = LEN(@mPO)
 WHILE(@lpo<>5 )
     BEGIN
        SET @mPurchaseOrderNumber += '0'
        SET @lpo = @lpo + 1
     END
SET @mPurchaseOrderNumber ='PO'+'-'+ CONVERT(NVARCHAR, YEAR(GETDATE())) + '-'+ @mPurchaseOrderNumber + CONVERT(nvarchar,@mPO)
RETURN @mPurchaseOrderNumber

it is just return a variable

 

Now I am calling it withing another store procedure with the following command

DECLARE @PON NVARCHAR(13)

EXEC UP_SELECT_PO_NUMBER @StoreBranchID = 4, @mPurchaseOrderNumber = @PON OUTPUT;

print @PON

and I am getting

 

"Conversion failed when converting the nvarchar value 'PO-2020-00023' to data type int." error. I have no type int in the calling procedure

 

Can someone help me spot my mistake?

 

Thanks in advance