SqlParameters Not Working With Stored Procedures

Anyone can help me why is this not working?

My function is

public void updateSupplierInformation(string FirstLetter, string Vendor, string SegmentTeam, string VendorRank, string ScrubOwner)

{

SqlCommand myCommand = new SqlCommand("sp_LAST_UpdateSupplier", myConnection);

 

myCommand.CommandType = CommandType.StoredProcedure;

myConnection.Open();

SqlParameter myParam;

myParam = myCommand.Parameters.Add(new SqlParameter("@FirstLetterS", SqlDbType.VarChar, 1));

myParam.Direction = ParameterDirection.Input;

myParam.Value = FirstLetter;

myParam = myCommand.Parameters.Add(new SqlParameter("@VendorS", SqlDbType.VarChar, 30));

myParam.Direction = ParameterDirection.Input;

myParam.Value = Vendor;

myParam = myCommand.Parameters.Add(new SqlParameter("@SegTeamS", SqlDbType.VarChar, 30));

myParam.Direction = ParameterDirection.Input;

myParam.Value = SegmentTeam;

myParam = myCommand.Parameters.Add(new SqlParameter("@VendorRankS", SqlDbType.VarChar, 30));

myParam.Direction = ParameterDirection.Input;

myParam.Value = VendorRank;

myParam = myCommand.Parameters.Add(new SqlParameter("@ScrubOwnerS", SqlDbType.VarChar, 30));

myParam.Direction = ParameterDirection.Input;

myParam.Value = ScrubOwner;

 

int i =myCommand.ExecuteNonQuery();

 

myConnection.Close();

}

My Stored procedure is:

ALTER PROCEDURE sp_LAST_UpdateSupplier

@FirstLetterS varchar(1),

@VendorS varchar(30),

@SegTeamS varchar(30),

@VendorRankS varchar(30),

@ScrubOwnerS varchar(30)

AS

UPDATE [gntm_user].LAST_Supplier_Ownership

SET First_Letter =@FirstLetterS,Seg_Team =@SegTeamS,Vendor_Rank =@VendorRankS,

Scrub_Owner =@ScrubOwnerS

WHERE Vendor=@VendorS

 

RETURN

 

When I debug on any of the myParam value it displays "@FirstLetterS,@VendorS" etc. not the value which is being passed in the function which is FirstLetter,Vendor.

Can anyone help me on this please? Thanks in advance