If `of` necessary when returning previous response with mergeMap?

I am chaining HTTP calls as per https://stackoverflow.com/a/34107312/470014 however I only want to execute the next call if the previous one returns null. So I have the following code:

public getTolerances() {
  this.queryServer(12,34).pipe(
    mergeMap(previous => {
        if (previous) {
            return of(previous);
        } else {
            return this.queryServer(0,34);
        }
    }),
    mergeMap(previous => {
        if (previous) {
            return of(previous);
        } else {
            return this.queryServer(34,0);
        }

    }),
    mergeMap(previous => {
        if (previous) {
            return of(previous);
        } else {
            return this.queryServer(0,0);
        }
    })
  )
  .subscribe((result) => {
      console.log({result});
  });
}

If the previous call returns a result I am using of operator to return that as an observable. Is that necessary or is there a way I can drop them? It feels like I am unwrapping the promise and then wrapping it up again but if I just return previous I get the following type error:

Argument of type '(result: unknown) => unknown' is not assignable to parameter of type '(value: unknown, index: number) => ObservableInput'. Type 'unknown' is not assignable to type 'ObservableInput'.ts(2345)