Typescript '；可观察<；任何>'；。财产'_isScalar'；类型'中缺少；NativeFs'；

vscode悬停“exists”会显示错误：

[ts]
Argument of type 'NativeFs' is not assignable to parameter of type 'Observable<any>'.
  Property '_isScalar' is missing in type 'NativeFs'.

interface NativeFs {
  <T>(...args: any[]): Rx.Observable<T>;
}

let readFile: NativeFs = Rx.Observable.bindNodeCallback(fs.readFile);
let exists: NativeFs = Rx.Observable.bindNodeCallback(fs.exists);

hierarchyPath$
  .takeUntil(exists) // <- "exists" error

src/entity/index.ts(27,14): error TS2345: Argument of type 'NativeFs' is not assignable to parameter of type 'Observable<any>'.
  Property '_isScalar' is missing in type 'NativeFs'.

tsc -v
Version 2.5.3