[SQL and JAVA] Having some trouble

public String vesselLookup(String VesselName){
                        String vessellookupnumber = "";
                        CallableStatement proc_stmt = null;
                        try{
                                proc_stmt = Global.con.prepareCall("{call sp_Vessellookup(?, ?)}");
                                proc_stmt.setString(1, VesselName);
                                proc_stmt.setInt(2, 0);
                        } catch (SQLException e1){
                                e1.printStackTrace();
                        }
                        try {
                                proc_stmt.registerOutParameter(2, Types.INTEGER);
                                proc_stmt.execute();
                                vessellookupnumber = Integer.toString(proc_stmt.getInt(2));
                                proc_stmt.close();
                        } catch (SQLException s) {
                                System.err.println(s);
                        }

                        return vessellookupnumber;
                }

USE [DEV_ACIMonitoring]
GO
/****** Object:  StoredProcedure [dbo].[sp_Vessellookup]    Script Date: 07/12/2012 11:23:35 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:              <XXXXXXXXXXX>
-- Create date: <2012-07-12>
-- Description: <Returns the value of the stored name, 
--                              If the name isnt in there, it Inserts a new row>
-- =============================================
ALTER PROCEDURE [dbo].[sp_Vessellookup](
        @vesselinput    varchar(200),
        @i                              int             OUTPUT
        )
AS
BEGIN
        DECLARE @vesselNumber   int
        SET @vesselNumber = (SELECT vesselNumber FROM tblVesselLookup WHERE vessel = @vesselinput)
    
        IF @vesselNumber IS NULL
        BEGIN
        DECLARE @maxVessels     INT SET @maxVessels = (SELECT (MAX(vesselNumber) + 1) FROM tblVesselLookup)
        IF @maxVessels = NULL SET @maxVessels = 1
        INSERT INTO tblVesselLookup VALUES (@maxVessels, @vesselinput)
        SET @i = @maxVessels
        END
        
        ELSE 
        BEGIN
        SET @i = @vesselNumber
        END
        
        RETURN @i
END

com.microsoft.sqlserver.jdbc.SQLServerException: Cannot insert the value NULL into column 'vesselNumber', table 'DEV_ACIMonitoring.dbo.tblVesselLookup'; column does not allow nulls. INSERT fails.