diff --git a/src/firestore/collection/collection.ts b/src/firestore/collection/collection.ts
index c1d6f3f0a..87730dfc2 100644
--- a/src/firestore/collection/collection.ts
+++ b/src/firestore/collection/collection.ts
@@ -154,7 +154,7 @@ export class AngularFirestoreCollection<T=DocumentData> {
    * Create a reference to a single document in a collection.
    * @param path
    */
-  doc<T>(path: string): AngularFirestoreDocument<T> {
+  doc<T>(path?: string): AngularFirestoreDocument<T> {
     return new AngularFirestoreDocument<T>(this.ref.doc(path), this.afs);
   }
 }