use HttpCompression show unreadable characters

Question

I want to use httpcompression in may asp.net 3.5 website and IIS 7.5 Server,so I Create a ClassLibrary(HttpCompression) and write this codes:

namespace Behsaman.Web
{
	public class HttpCompression : IHttpModule
	{
		#region IHttpModule Members

		public void Dispose()
		{
		}

		void IHttpModule.Init(HttpApplication context)
		{
			context.BeginRequest += new EventHandler(context_BeginRequest);
		}

		void context_BeginRequest(object sender, EventArgs e)
		{

			HttpApplication app = (HttpApplication)sender;

			string encodings = app.Request.Headers.Get("Accept-Encoding");
			if (!app.Request.RawUrl.Contains(".aspx") ||
				HttpContext.Current.Request["HTTP_X_MICROSOFTAJAX"] != null ||
				HttpContext.Current.Request["http_x-microsoftajax"] != null ||
				HttpContext.Current.Items["hasBeenCompressed"] !=null ||
				string.IsNullOrEmpty(encodings))
				return;
			encodings = encodings.ToLower();

      if (encodings.Contains("deflate"))
      {
        app.Response.Filter = new DeflateStream(app.Response.Filter, CompressionMode.Compress);
        app.Response.AppendHeader("Content-Encoding", "deflate");
      }
      else if (encodings.Contains("gzip"))
			{

				app.Response.Filter = new GZipStream(app.Response.Filter, CompressionMode.Compress);
				app.Response.AppendHeader("Content-Encoding", "gzip");
			}
      else
      {
        return;
      }
			app.Response.Cache.VaryByHeaders["Accept-encoding"] = true;
			HttpContext.Current.Items["hasBeenCompressed"] = true;
		}

		#endregion
	}
}

and then add this dll to my website refrences. then I write this code in web.config

# < system.webServer >  
# ......... 
#   < modules >  
#    ........ 
#    < add  name = "HttpCompression"  type = "Behsaman.Web.HttpCompression" />  
#   </ modules >  
#  ............. 
#  </ system.webServer >

when I browse a page I see unreadable characters like this:

�`I�%&/m�{

I see firebug and check Response Header :

Cache-Control	private
Content-Type	text/html; charset=utf-8
Content-Encoding	deflate,
Server	Microsoft-IIS/7.5
X-AspNet-Version	2.0.50727
X-Powered-By	ASP.NET
Date	Sun, 05 Dec 2010 08:08:13 GMT
Content-Length	62877

In the Content-Encoding row you can see a comma after defalte word, is it normal?How can I fix my problem?