vue-test-utils | TypeError: s.split is not a function

I try to run a test with vue-test-utils, including a component that has a mixin included, which has a function using split() on a string. The test looks like this:

describe('adminExample.vue Test', () => {
    const wrapper = shallowMount(adminExample, {
        global: {
            mixins: [globalHelpers, authGatewayForElements, storeService],
            plugins: [store]
        }
    })
    it('renders component and is named properly', () => {
        // check the name of the component
        expect(wrapper.vm.$options.name).toMatch('adminExample')
    })
})

adminExample.vue doesn't give any error, so I don't include it here, bit it uses a mixin.

The included mixin, called authGatewayForElements, has a function called decryptToken() and simply decrypt a jwt to get some info. The parameter userToken is declared within data of this mixin. The function looks like this:

decryptToken() {
            let base64Split = this.userToken.split('.')[1];
            let base64 = base64Split.replace(/-/g, '+').replace(/_/g, '/');
            let jsonPayload = decodeURIComponent(atob(base64).split('').map(function(c) {
                return '%' + ('00' + c.charCodeAt(0).toString(16)).slice(-2);
            }).join(''));
            return JSON.parse(jsonPayload);
        },

Running the test giving me the error TypeError: this.userToken.split is not a function . I´m new to testing with vue-test-utils and maybe or definitely missing something that needs to beincluded in the wrapper, as I expected functions like split() don't need to be included additionally.

EDIT: I get this error on multiple functions, like find(), so I'm pretty sure I just do something wrong. Thanks in advance to anybody pointing that out.