error TS2415: Class 'Subject<T>' incorrectly extends base class 'Observable<T>'.

Esta tudo funcionando mas tem um warning durante a compilação (npm start do client). Mensagem no terminal (console do SO):

17:24:59 - File change detected. Starting incremental compilation...


node_modules/rxjs/Subject.d.ts(16,22): error TS2415: Class 'Subject<T>' incorrectly extends base class 'Observable<T>'.
  Types of property 'lift' are incompatible.
    Type '<T, R>(operator: Operator<T, R>) => Observable<T>' is not assignable to type '<R>(operator: Operator<T, R>) => Observable<R>'.
      Type 'Observable<T>' is not assignable to type 'Observable<R>'.
        Type 'T' is not assignable to type 'R'.
17:24:59 - Compilation complete. Watching for file changes.