graphql-voyager: Cannot find module 'koa'.

When importing as mentioned in docs for an express project:

import { express as voyagerMiddleware } from 'graphql-voyager/middleware';

I get

 node_modules/graphql-voyager/middleware/koa.d.ts:1:25 - error TS2307: Cannot find module 'koa'.

 1 import { Context } from 'koa';

from typescript@2.9.2