Mocha ES6 Babel "TypeError: undefined is not a function"

I'm currently trying to implement simple tests for my JavaScript components and I am getting the error: TypeError: undefined is not a function from my test.

This is erroring on this line within my class:

this.options = Object.assign({}, this.defaults, options);

this.defaults is an object of default options and options is currently an object with nothing in.

I am running the code from CLI using npm test which resolves to this: mocha ./src/components/myComponent/myComponent.spec.js --compilers js:babel-register

Does anyone know why this error occurs?