Adding a tsconfig file breaks ts-jest?

[commure-stabai]

I have some code generated by Swagger Codegen, and I'm trying to publish it as an npm package with both JavaScript and TypeScript support. I had all of my ts-jest tests passing before I had created a tsconfig.json file, but when I added one, I started getting errors.

The big error is actually pointing to the file under test, saying TypeError: f is not a function:

  MyServiceApi
    ✕ getThing (4 ms)

  ● MyServiceApi › getThing

    TypeError: f is not a function

      536 |             return (f: FetchAPI = isomorphicFetch, basePath: string = BASE_PATH) => {
      537 |                 // @ts-ignore
    > 538 |                 return f(basePath + localVarFetchArgs.url, localVarFetchArgs.options).then((response) => {
          |                        ^
      539 |                     if (response.status >= 200 && response.status < 300) {
      540 |                         return response.json();
      541 |                     } else {

      at api.ts:538:24
      at MyServiceApi.getThing (api.ts:585:76)
      at Object.<anonymous> (api_test.spec.ts:38:28)

The definition of the FetchAPI interface looks right, and tsc is perfectly happy:

export interface FetchAPI {
    (url: string, init?: any): Promise<Response>;
}

I had to change some imports when adding the tsconfig, so I thought that could be it, but none of the changes have made any difference on this error.

What am I missing?