Error: criticalSrc.indexOf is not a function When Trying to Run Production Build

[brianrivet-tilt]

Hi,

I'm trying to use the laravel-mix-criticalcss extension on a site I'm working on that uses Mix for the front end and I'm getting the following error in my build process when it tries to run it:

TypeError: criticalSrc.indexOf is not a function
at /node_modules/laravel-mix-criticalcss/index.js:39:33
at Array.forEach ()
at Critical.webpackPlugins (/buddy/investarstage/node_modules/laravel-mix-criticalcss/index.js:34:30)

Here's my code from my mix file:

.criticalCss({
		enabled: mix.inProduction(),
		paths: {
			base: process.env.PRIMARY_SITE_URL,
			templates: './css/critical', //Where css files need to be written, all these paths are relative to /public
										 //So the example path here will be public/css/critical       
			suffix: '_critical.min'
		},
		urls: [
			{ url: process.env.BASE_URL , template: 'index' },
			{ url: process.env.BASE_URL + 'personal-banking', template: 'personal' },
			{ url: process.env.BASE_URL + 'small-business-banking', template: 'smallbusiness' },
			{ url: process.env.BASE_URL + 'commercial-banking', template: 'commercial' },
			{ url: process.env.BASE_URL + 'about-us', template: 'about' },
			{ url: process.env.BASE_URL + 'locations', template: 'location' },			
		],
		//Now using https://github.com/addyosmani/critical v4.0.1
		options: {
			//It's important to note here you should NOT set inline:true, this will break the whole system.
			
			minify: true,
			dimensions: [
				{
				  height: 700,
				  width: 300,
				},
				{
				  height: 200,
				  width: 500,
				},
				{
				  height: 1024,
				  width: 768,
				},
				{
				  height: 900,
				  width: 1024,
				},
			  ],
			
		},
	})

Can you help me to troubleshoot why this is occurring?