jsii does not reject the following interface definition:

export interface Thingamajig {
    [key: string]: string
}

But what it generates for it is an empty interface, which is not correct:

    "@scope/lib.Thingamajig": {
      "assembly": "@scope/lib",
      "fqn": "@scope/lib.Thingamajig",
      "kind": "interface",
      "name": "Thingamajig"
    },

It should either reject this definition, or generate something sensible.