Angular2/Ionic2: No provider for ….

When your developing an angular 2/ionic 2 application and you get the the following error: EXCEPTION: Error: Uncaught (in promise): EXCEPTION: Error in :0:0ORIGINAL EXCEPTION: No provider for UserService!ORIGINAL STACKTRACE:Error: DI Exception    at NoProviderError.BaseException [as constructor] (http://localhost:8100/build/js/app.bundle.js:2300:23)    at NoProviderError.AbstractProviderError [as constructor] (http://localhost:8100/build/js/app.bundle.js:27491:16)    at new NoProviderError (http://localhost:8100/build/js/app.bundle.js:27528:16)    at ReflectiveInjector_._throwOrNull (http://localhost:8100/build/js/app.bundle.js:28516:19)    at ReflectiveInjector_._getByKeyDefault (http://localhost:8100/build/js/app.bundle.js:28544:25)    at ReflectiveInjector_._getByKey (http://localhost:8100/build/js/app.bundle.js:28507:25)   
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