While @silent__thought's solution may correct the problem, it's not the easiest way to fix it.
@Costas solution may exactly address the problem, but here is a more detailed explanation if you need it.
In my case, I needed the following in my web.config file (for MVC 4):
<configuration>
<appSettings>
<add key="webpages:Version" value="2.0.0.0" />
<add key="webpages:Enabled" value="false" />
<add key="PreserveLoginUrl" value="true" />
<add key="ClientValidationEnabled" value="true" />
<add key="UnobtrusiveJavaScriptEnabled" value="true" />
</appSettings>
<system.web>
<compilation debug="true" targetFramework="4.0" />
<pages>
<namespaces>
<add namespace="System.Web.Helpers" />
<add namespace="System.Web.Mvc" />
<add namespace="System.Web.Mvc.Ajax" />
<add namespace="System.Web.Mvc.Html" />
<add namespace="System.Web.Routing" />
<add namespace="System.Web.WebPages" />
</namespaces>
</pages>
<httpHandlers></httpHandlers>
</system.web>
</configuration>
Of course, you may not need all of these configurations for your app. These just happen to be the default settings for an MVC 4 project.