This is expression is not callable when adding a module to Nuxt, why?

Question

typescript

Added a project to nuxt, module moment-module , installed according to the manual in readme.md
However, when used in a component ::::

...
    computed: {
      currentMonth (): string {
        return this.$moment(this.viewableDate).format('mmmm')
      },

      daysInMonth (): number {
        return this.$moment(this.viewableDate).daysInMonth()
      }
    }
...

I am getting an error in this.$moment(...) expression

This expression is not callable.
  Not all constituents of type '(() => any) | ComputedOptions<any>' are callable.
    Type 'ComputedOptions<any>' has no call signatures.Vetur(2349)