title: DnaMarshalException when running Excel DNA FSharp Registration Sample
body: <p>I'm trying to follow <a href="https://github.com/Excel-DNA/Registration/blob/master/Source/Samples/Registration.Sample.FSharp" rel="nofollow">the sample project</a> for using Excel-DNA Registration Helpers for F Sharp. In particular, I'm interested in the sample for using optional arguments.</p>
  
  <p>I've built the sample project (using Visual Studio 2015), but when I load the resulting Xll into Excel 2010 I receive the error</p>
  
  <blockquote>
    <p>Registration [Error] Method not registered due to unsupported signature: 'Func`4.Invoke' : DnaMarshalException - Unknown Data Type: Microsoft.FSharp.Core.FSharpOption`1[System.Double]</p>
  </blockquote>
  
  <p>in the diagnostics window, and the function <code>dnaFSharpOptional</code> is unavailable in Excel. It's as if the parameter conversion <code>FsParameterConversions.FsOptionalParameterConversion</code> is not being applied or is not working as intended.</p>
  
  <p>Any ideas of the source of the error, and how to fix it?</p>
  
  <p>N.B.</p>
  
  <ol>
  <li>As I'm only building the sample project, and not the whole Excel-DNA Registration solution, I had to change some of the project references (namely ExcelDna.Registration, ExcelDna.Registration.FSharp) to point to pre-built assemblies from nuget. But I don't imagine this would make a difference.</li>
  <li>The sample project references Excel 2013 in the project Debug settings but I'm using Excel 2010.</li>
  <li>The other example functions in the project (i.e. the Async functions) are available, so it's not a case of registration failing generally - it's something specific to the optional arguments.</li>
  </ol>