Web Pages exception in ASP.NET MVC 3 application after installing MVC 4 Beta

After installing ASP.NET MVC 4 Beta I tried to run one of my MVC 3 applications and got this error:

System.InvalidCastException: [A]System.Web.WebPages.Razor.Configuration.HostSection cannot be cast to [B]System.Web.WebPages.Razor.Configuration.HostSection. Type A originates from 'System.Web.WebPages.Razor, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' in the context 'Default' at location 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Razor\v4.0_1.0.0.0__31bf3856ad364e35\System.Web.WebPages.Razor.dll'. Type B originates from 'System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' in the context 'Default' at location 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Razor\v4.0_2.0.0.0__31bf3856ad364e35\System.Web.WebPages.Razor.dll'.

For some reason the framework was loading version 2.0 of System.Web.WebPages.Razor.dll instead of version 1.0. To fix it I just set webpages:Version to 1.0 in Web.config:

<appSettings>
   <add key="webpages:Version" value="1.0"/>
</appSettings>