Trouble with plugin not firing on create of Order Products

I am attempting to create a plugin that will set the Quantity of Order Products to 1 on initial create of an order.  The reason I need to do this is the end users attach Opportunity Products to an Opportunity, then create an order from the Opportunity for "Free" copies of a product and the client doesn't want to give away many free copies by mistake.

The problem I have is that the plugin does not fire when I register it for Create on salesorderdetail (pre operation).  Is there something simple I'm doing wrong?

My code is:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
// Microsoft Dynamics CRM namespace(s)
using Microsoft.Xrm.Sdk;
using System.ServiceModel;

namespace CRMPlugin1
{
    public class SetOrderProductQty : IPlugin
    {
        public void Execute(IServiceProvider serviceProvider)
        {

            // Obtain the execution context from the service provider.
            IPluginExecutionContext context = (IPluginExecutionContext)
            serviceProvider.GetService(typeof(IPluginExecutionContext));


            // The InputParameters collection contains all the data passed in the message request.
            if (context.InputParameters.Contains("Target") &&
            context.InputParameters["Target"] is Entity)
            {
                // Obtain the target entity from the input parmameters.
                Entity entity = (Entity)context.InputParameters["Target"];

                try
                {

                    
                    if (entity.Attributes.Contains("quantity") == true)
                    {
                        entity.Attributes["quantity"] = 1;
                    }
                    else
                    {
                        entity.Attributes.Add("quantity", 1); 
                    }
                                       }
                catch (FaultException ex)
                {
                    throw new InvalidPluginExecutionException("An error occurred in the plug-in.", ex);
                }

            }

        }
    }//end class
}//end name space