Unable to use extension method within partial view

I am trying to use a string extension method in a partial view. I get the following error:

'string' does not contain a definition for 'TruncateAtCharacter'

Here is the extension method:

namespace PCCMS.Core.Libraries {
    public static class Extensions {
        public static string TruncateAtCharacter(this string input, int length) {
            if (String.IsNullOrEmpty(input) || input.Length < length)
                return input;

            return string.Format("{0}...", input.Substring(0, length).Trim());
        }
    }
}

According to this previous question I need to add the namespace to web.config, however I have done this and I still receive the same error message. What's odd though, is that I do get intellisense for the extension method?

<system.web.webPages.razor>
    <host factoryType="System.Web.Mvc.MvcWebRazorHostFactory, System.Web.Mvc, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
    <pages pageBaseType="PCCMS.Core.Libraries.ClientWebViewPage">
        <namespaces>
            <add namespace="System.Web.Mvc" />
            <!-- Other namespaces... -->
            <add namespace="PCCMS.Core.Libraries" />
        </namespaces>
    </pages>
</system.web.webPages.razor>

Can anyone explain why this is?

Thanks