accepting a sql stored procedure int parameter as NULL

Hi im new to C#, this is driving me crazy, any help would be appreciated..

I have a sql stored proc which accepts parameters;

CREATE PROCEDURE Sp_GetGamingDefaults

(@ReportMode int, @ModuleUID int, @LoadNulls bit) AS  blah

in c#.net the ReportMode can be null, I keep getting this error;

{"Procedure or function 'Sp_GetGamingDefaults' expects parameter '@ReportMode', which was not supplied."}

here is the code, im sure im missing something simple.... im setting the @ReportMode as SqlDbType.Int

try

{

SqlCommand acommand = connection.CreateCommand();

acommand.CommandText = "BIM.dbo.Sp_GetGamingDefaults";

acommand.CommandType = CommandType.StoredProcedure;

acommand.Parameters.Add("@ReportMode", SqlDbType.Int);

acommand.Parameters.Add("@ModuleUID", SqlDbType.Int);

acommand.Parameters.Add("@LoadNulls", SqlDbType.Bit);

if (ReportMode == null)

{

acommand.Parameters["@ReportMode"].IsNullable = true;

acommand.Parameters["@LoadNulls"].Value = true;

}

else

{

 acommand.Parameters["@LoadNulls"].Value = false;

}

acommand.Parameters["@ReportMode"].Value = ReportMode;

acommand.Parameters["@ModuleUID"].Value = ModuleUID;

acommand.Prepare();

SqlDataAdapter adapt = new SqlDataAdapter(acommand.CommandText, connection);

adapt.TableMappings.Add("Table", "defaults");

adapt.SelectCommand = acommand;

adapt.Fill(aDataSet);

acommand.Dispose();

adapt.Dispose();

}

catch (Exception e)

{

MessageBox.Show(e.Message);

}