WinRTError: Unknown runtime error when referencing a c# component from javascript

I'm attempting to access a C# component in UWP, but I get an unhelpful error when doing so: WinRTError: Unknown runtime error. Why does this error occur, and how can I stop it?

I'm using Windows 10, VS 2017, and building my project to meet the minimum version of 10.0.16299.0

Here's a MVCE showing the behavior: https://github.com/re-jim/UWPComponentSample

The C# component is set as a reference in my Javascript component.

Javascript:

The below function triggers the C# component, which should zip a directory

async zip(content) {
    try {
        console.log('ZipComponent is ', ZipComponent)
        console.log('function is ', ZipComponent.ZipWorker.zipDirectory)
        ZipComponent.ZipWorker.zipDirectory(content.dirPath, content.dirPath + '\\file.ezdrm')
    } catch (err) {
        console.log('err is ',err)
    }
}

I've console.logged the component and the method I intend to use to show that exists:

C#

using System;
using System.Diagnostics;
using System.IO.Compression;

namespace ZipComponent
{
    public sealed class ZipWorker
    {

        public static void ZipDirectory(string folderPath, string outputFile)
        {
            try
            {
                Debug.WriteLine("zippping " + folderPath + " output " + outputFile);
                ZipFile.CreateFromDirectory(folderPath, outputFile);
            }
            catch (Exception e)
            {
                Debug.Write(e);
            }

        }

        public static void UnzipDirectory(string zipFile, string outputPath)
        {
            try
            {
                Debug.WriteLine("unzippping " + zipFile + " output " + outputPath);
                ZipFile.ExtractToDirectory(zipFile, outputPath);
            }
            catch (Exception e)
            {
                Debug.Write(e);
            }
        }
    }
}

I don't see the output from Debug.WriteLine in my output

The code inside of ZipDirectory() doesn't seem to be causing the error. I replaced the function with:

public static string ZipDirectory(){

    return "hello world"
}

after also editing the javascript function call to match, And that gives me the same error as below.

Stack trace:

"WinRTError: Unknown runtime error
   at ZipService.prototype.zip (ms-appx://9ecba615-ddcd-4c3c-a8f1-a09e9aac3e60/dist/zip.service.js:12:13)
   at Generator.prototype.next (native code)
   at onZipRequest (ms-appx://9ecba615-ddcd-4c3c-a8f1-a09e9aac3e60/dist/wrapperMain.js:69:9)
   at Generator.prototype.next (native code)
   at SafeSubscriber.prototype.__tryOrUnsub (ms-appx://9ecba615-ddcd-4c3c-a8f1-a09e9aac3e60/dist/rxjs/rxjs.umd.js:477:13)
   at SafeSubscriber.prototype.next (ms-appx://9ecba615-ddcd-4c3c-a8f1-a09e9aac3e60/dist/rxjs/rxjs.umd.js:415:17)
   at Subscriber.prototype._next (ms-appx://9ecba615-ddcd-4c3c-a8f1-a09e9aac3e60/dist/rxjs/rxjs.umd.js:359:9)
   at Subscriber.prototype.next (ms-appx://9ecba615-ddcd-4c3c-a8f1-a09e9aac3e60/dist/rxjs/rxjs.umd.js:336:13)
   at Subject.prototype.next (ms-appx://9ecba615-ddcd-4c3c-a8f1-a09e9aac3e60/dist/rxjs/rxjs.umd.js:755:17)
   at AppShellService.prototype.zipContent (ms-appx://9ecba615-ddcd-4c3c-a8f1-a09e9aac3e60/dist/main.js:197:13)"