Return type for the stored procedure could not be detected.

When I try to put my SP into DBML then it gives the following error.

the return types for the following stored procedures could not be detected.Set the return type for each stored procedure in the Properties window. 

USE [PROJECT]
GO
/****** Object:  StoredProcedure [dbo].[Insert]    Script Date: 27.05.2013 17:30:27 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER OFF
GO
ALTER PROCEDURE [dbo].[Insert]

@NT_Id varchar(100),
@projektbezeichnung varchar(100),
@verrechenbar numeric(5,2),
@pa_name varchar(30),
@datum smalldatetime



AS
BEGIN

insert into MZSGMitarbeiter(NT_id) values (@NT_Id)
insert into BusinessCase(projektbezeichnung) values(@projektbezeichnung)
insert into LeistungsErfassung(verrechenbar) values (@verrechenbar)
insert into ProjektAuftrag(name) values(@pa_name)
insert into LeistungsErfassung(datum) values (@datum)

    
END