Web.config pollingDuplex error

Hi, i'm doing a polling duplex service and have this web.config

<?xml version="1.0"?>

<!--
  For more information on how to configure your ASP.NET application, please visit
  http://go.microsoft.com/fwlink/?LinkId=169433
  -->

<configuration>
    <system.web>
        <compilation debug="true" targetFramework="4.0" />
    </system.web>

  <system.serviceModel>
    <extensions>
      <behaviorExtensions>
        <add name="pollingDuplex" type="System.ServiceModel.Configuration.PollingDuplexHttpBindingCollectionElement, System.ServiceModel.PollingDuplex"/>
      </behaviorExtensions>
    </extensions>
    <behaviors>
      <serviceBehaviors>
        <behavior name="">
          <serviceMetadata httpGetEnabled="true"/>
          <serviceDebug  includeExceptionDetailInFaults="false"/>
        </behavior>
      </serviceBehaviors>
    </behaviors>
    <bindings>
      <pollingDuplex>
        <binding name="myPollingDuplex" duplexMode="MultipleMessagesPerPoll" />
      </pollingDuplex>
    </bindings>
    <serviceHostingEnvironment aspNetCompatibilityEnabled="true" multipleSiteBindingsEnabled="true" />
    <services>
      <service name="ChatDemo.Web.ChatService">
        <endpoint address="" binding="pollingDuplex" bindingConfiguration="myPollingDuplex" contract="ChatDemo.Web.ChatService" />
        <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />
      </service>
    </services>
  </system.serviceModel>
</configuration>

and i'm getting this error: Parser Error Message: Configuration binding extension 'system.serviceModel/bindings/pollingDuplex' could not be found. Verify that this binding extension is properly registered in system.serviceModel/extensions/bindingExtensions and that it is spelled correctly.

Re: Web.config pollingDuplex error

Forget it...typing error! x.x

i put <behaviorExtensions> instead of <bindingExtensions>

lol!