How can you test a Vue computed property without mounting?

I am trying to test a computed property that is dependent on several other computed properties of a Vue instance. I can access the wrapper's vm.normalizedHeadwind property in the spec...

const wrapper = getWrapper()  // helper that returns wrapper with test data;

let result = (wrapper.vm as any).normalizedHeadwind; // returns expected value

but if I try:

ExceedenceCard.computed.normalizedHeadwind.call({... dependency data})

as described in https://lmiller1990.github.io/vue-testing-handbook/computed-properties.html#testing-by-rendering-the-value

I get undefined for the ExceedenceCard.computed property.

ExceedenceCard.ts

export default Vue.extend({
  ...
  computed: {
    normalizedHeadwind: {
      get: function(): number {
        const { direction, speed } = this.currentWindConditions;
        return this.rounded(
          calculateHeadwind(this.runwayHeading, direction, speed)
        );
      }
    },
  ...

I've tried refactoring the code from using the vue-class-component and vue-property-decorator modules back to using the Vue.extend pattern and it made no difference.