Unable to run an asp.net 4.5 app on xsp on Mono 3

I've build Mono 3.0.2 from source (tarball), and built XSP from both the latest tarball and the latest on Github, but I'm unable to run a relatively simple asp.net app using .net 4.5 because it sees 'targetFramework="4.5"' in the web.config as invalid. Building the app, and running a console .net 4.5 app works just fine.

This is the web.config in question:

<?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>
  <appSettings>
    <add key="owin:HandleAllRequests" value="true" />
    <add key="owin:SetCurrentDirectory" value="true" />
  </appSettings>

  <system.web>
    <compilation debug="true" targetFramework="4.5" />
    <httpRuntime targetFramework="4.5" />
  </system.web>

</configuration>

And this is the exception that xsp4 is throwing:

An exception has occurred while generating HttpException page:
System.NullReferenceException: Object reference not set to an instance of an object
  at System.Web.Util.HttpEncoder.GetCustomEncoderFromConfig () [0x00000] in <filename unknown>:0
  at System.Lazy`1[System.Web.Util.HttpEncoder].InitValue () [0x00000] in <filename unknown>:0

The actual exception which was being reported was:
System.Web.HttpException: Initial exception ---> System.Configuration.ConfigurationErrorsException: Error deserializing configuration section httpRuntime: Unrecognized attribute 'targetFramework'. (/home/srobbins/Projects/nancykatana/NancyKatana/Web.config line
1)
  at System.Configuration.ConfigurationSection.DeserializeSection (System.Xml.XmlReader reader) [0x00000] in <filename unknown>:0
  at System.Configuration.Configuration.GetSectionInstance (System.Configuration.SectionInfo config, Boolean createDefaultInstance) [0x00000] in <filename unknown>:0
  at Syst