foreign key relationships on delete cascade on update cascade - sql

Question

In sql server 2012 , I have table PurchaseReturn(VendorId,PurchaseOrderId). The column 'Vendorid' is a foreign key relationship to Vendor(VendorId) and 'Purchaseorderid' is to Purchaseorder(purchaseorderi).
Now my target is to set type of foreign key for 'VendorId' as 'On delete Set Null on Update Cascade' but For 'purchaseorderid' it should be 'On delete CasCade On Update Cascade'.
But when i have created FK for VendorId and trying to make it for PurchaseOrderId by following query:
alter table PurchaseReturn add constraint FK_PR_PORD foreign key (Purchaseorderid) references PurchaseOrder(Purchaseorderid) on delete cascade ON UPDATE CASCADE ';
an errors appears like this:
Introducing FOREIGN KEY constraint 'FK_PR_PORD' on table 'PurchaseReturn' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
What is the problem?... plz help thanks in advance
This is the create table query alonq with constraints
USE [AsifTraders]
GO
/****** Object: Table [dbo].[PurchaseReturn] Script Date: 24-Sep-14 11:46:39 AM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[PurchaseReturn](
[Date] [datetime] NOT NULL,
[PurchaseReturnId] [nvarchar](20) NOT NULL,
[VendorId] [nvarchar](8) NULL,
[Description] [nvarchar](50) NOT NULL,
[ReturnType] [nvarchar](20) NOT NULL,
[Deduction] [numeric](15, 0) NULL,
[ProductQuantity] [numeric](10, 2) NOT NULL,
[PurchaseValue] [numeric](12, 0) NOT NULL,
[ReturnValue] [numeric](15, 0) NOT NULL,
[Paid] [numeric](15, 0) NULL,
[PBalance] [numeric](15, 0) NULL,
[NBalance] [numeric](15, 0) NULL,
[JEntryId] [int] NOT NULL,
[PURCHASEORDERID] [nvarchar](8) NULL,
CONSTRAINT [PK_PurchaseReturn] PRIMARY KEY CLUSTERED
(
[PurchaseReturnId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[PurchaseReturn] WITH CHECK ADD CONSTRAINT [FK_PR_VEN] FOREIGN KEY([PURCHASEORDERID])
REFERENCES [dbo].[PurchaseOrder] ([PurchaseOrderId])
ON UPDATE CASCADE
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[PurchaseReturn] CHECK CONSTRAINT [FK_PR_VEN]
GO
This is the query for PurhcaseOrder
USE [AsifTraders]
GO
/****** Object: Table [dbo].[PurchaseOrder] Script Date: 24-Sep-14 11:56:53 AM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[PurchaseOrder](
[Date] [datetime] NULL,
[PurchaseOrderId] [nvarchar](8) NOT NULL,
[JEntryId] [int] NULL,
[VendorId] [nvarchar](8) NULL,
[PurchaseType] [nvarchar](10) NULL,
[Description] [nvarchar](50) NOT NULL,
[ItemQuantity] [numeric](18, 0) NOT NULL,
[DeleiveryCharges] [numeric](18, 0) NULL,
[Discount] [numeric](18, 0) NULL,
[Total] [numeric](18, 0) NOT NULL,
[Paid] [numeric](18, 0) NULL,
[PBalance] [numeric](18, 0) NULL,
[NBalance] [numeric](18, 0) NULL,
PRIMARY KEY CLUSTERED
(
[PurchaseOrderId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[PurchaseOrder] WITH CHECK ADD CONSTRAINT [fk_purch_ven] FOREIGN KEY([VendorId])
REFERENCES [dbo].[Vendor] ([VendorId])
ON UPDATE CASCADE
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[PurchaseOrder] CHECK CONSTRAINT [fk_purch_ven]
GO