Stored Procedure Not Updating

Using VS.Net 2003 and C#

Why won't this update the table?

public bool changeVisibility( string Id, bool visibility )

{

using( SqlConnection conn = new SqlConnection( @SQLHelper.CONN_STRING ) )

{

using( SqlCommand command = new SqlCommand( "changeVisibility", conn ) )

{

command.CommandType = CommandType.StoredProcedure;

command.Parameters.Add( "@Id", Id );

command.Parameters.Add( "@visibility", visibility );

<snip>

And then the stored procedure:

CREATE PROCEDURE dbo.changeVisibility

 @Id           VARCHAR(20), 
 @visibility         BIT


 AS
BEGIN
 BEGIN TRAN  
  
  UPDATE courseStructure
  SET    visible = @visibility                     
  WHERE  Id = @Id  

 COMMIT TRAN

 END
GO

The @visibility seems to be the problem.

I set it in the code as boolean.  And it goes into the sp as a BIT.

This should work but doesn't.

If I change the SET visible = 0 it works ok, but not with @visibility.

Thanks,

Zath