Can't resolve all parameters for AngularFirestore: ([object Object], ?)

I've been trying to troubleshoot this error for a while. The error reads Can't resolve all parameters for AngularFirestore: ([object Object], ?). Has anyone else had this issue, and how were you able to resolve the issue. I've read the docs, and I can't get to the bottom of this issue.

import { Component } from '@angular/core';
import { AngularFirestore } from 'angularfire2/firestore';
import { AngularFireDatabase } from 'angularfire2/database';
import { Observable } from 'rxjs/Observable';

@Component({
selector: 'app-root',
templateUrl: './app.component.html',
styleUrls: ['./app.component.css'],
providers: [AngularFirestore, AngularFireDatabase]
})
export class AppComponent {
items: Observable<any[]>;

constructor(db: AngularFirestore, adb: AngularFireDatabase) {
this.items = db.collection('0').valueChanges();
console.log(this.items)

}
}


package.json

"angularfire2": "^5.0.0-rc.3",
"firebase": "^4.6.0",

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