ngrx/effects unit tests using jasmine-marbles

When I try to use either cold or hot functions from jasmine-marbles, I keep getting TypeError: Class constructor Observable cannot be invoked without 'new'. Anybody know how to resolve this error? Here is what I have below.

   import { hot, cold } from "jasmine-marbles";
   import { Observable } from "rxjs/Observable";

   const myAction = hot("--a-", { a: "TEST" };