workflowservices.js Error (get_context) when attempting to start 2013 Workflow via JavaScript

I’m attempting to run the code provided in this thread:
Launch SharePoint 2013 Workflow w/Javascript

It’s a function which starts a SP 2013 Workflow given input parameters of Item ID and Subscription ID.

I keep getting an error on this line:

var subscription = wfManager.getWorkflowSubscriptionService().getSubscription(subscriptionId);

I keep getting a TypeError error:

this.get_context is not a function

Any ideas?

I’m attempting to run this from a web part. I see that sp.js, sp.core.js, and sp.runtime.js all appear to load by default.

The only one which I had to load with a script tag is sp.workflowservices.js.

My startWorkflow() function runs upon clicking a link, so I don’t think it’s a matter of code executing before scripts are properly loaded.

var clientContext = SP.ClientContext.get_current();
var workflowServicesManager = SP.WorkflowServices.WorkflowServicesManager.newObject(clientContext, clientContext.get_web());