Probem With Stored Procedure?

Trying to tackle this problem. It's a stored procedure that modifies data in two tables with a one-to-many-to-one relationship. I'm creating a C# front-end to change the data fields (Table1Data and Table2Data) in ASP.NET 2.0/VS 2005.

The error I get is "Procedure or function Modify has too many arguments specified. " when I try to edit.

 Table1 has Table1ID (PK, int), Table1Data (varchar(50)) ; Table2 has Table2ID (PK, int), Table2Data (varchar(50)) ; Table1_2 has Table1_2ID (PK,int), Table1ID (FK,int), Table2ID(FK,int)

There is a cascade upon UPDATE and DELETE one-to-many between Table1.Table1ID and Table1_2.Table1ID. Likewise, for Table2ID.

Here's my (not working) stored procedure for changing Table1Data and Table2Data at the same time. Each of the SQL blocks work fine, independently:
ALTER PROCEDURE [dbo].[Modify]
      (
 @Table1ID int,
 @Table1Data varchar(50),
 @Table2Data varchar(50)
      )
AS
BEGIN
      SET NOCOUNT ON;
     
UPDATE    Table1
SET              Table1Data = @Table1Data
WHERE     (Table1ID = @Table1ID)
 
UPDATE    Table2
SET              Table2Data = @Table2Data
FROM         Table2 INNER JOIN
                      Table1_2 ON Table2.Table2ID = Table1_2.Table2ID INNER JOIN
                      Table1 ON Table1_2.Table1ID = Table1.Table1ID
WHERE     (Table1.Table1ID = @Table1ID)
 
END

Many thanks for any help!