The "file" argument must be of type string. Received null

[lukemorcom]

Hi, I get the above error when trying to run the example png.

The full trace:

(node:8756) UnhandledPromiseRejectionWarning: TypeError [ERR_INVALID_ARG_TYPE]: The "file" argument must be of type string. Received null
    at new NodeError (internal/errors.js:322:7)
    at validateString (internal/validators.js:124:11)
    at normalizeSpawnArguments (child_process.js:486:3)
    at spawn (child_process.js:653:13)
    at Object.spawnWithSignal [as spawn] (child_process.js:911:17)
    at C:\Users\Luke\Documents\VT\wave\waveform.js\lib\index.js:91:20
    at new Promise (<anonymous>)
    at Waveform.waveform (C:\Users\Luke\Documents\VT\wave\waveform.js\lib\index.js:70:20)
    at C:\Users\Luke\Documents\VT\wave\index.js:11:8
    at Layer.handle [as handle_request] (C:\Users\Luke\Documents\VT\wave\node_modules\express\lib\router\layer.js:95:5)

Do you know why this may be? I noticed that your instructions require npm install but there are no dependencies listed in package.json. Am I missing something here?

Thanks in advance, mate :)