Corrected misleading error message when doc() is called with undefined.

Author: cherylEnkidu

packages/firestore/src/lite-api/reference.ts

@@ -627,6 +627,12 @@ export function doc<AppModelType, DbModelType extends DocumentData>(
   path?: string,
   ...pathSegments: string[]
 ): DocumentReference<AppModelType, DbModelType> {
+  if (parent === undefined) {
+    throw new FirestoreError(
+      Code.INVALID_ARGUMENT,
+      'Function doc() cannot be called with an undefined first argument.'
+    );
+  }
   parent = getModularInstance(parent);
 
   // We allow omission of 'pathString' but explicitly prohibit passing in both
@@ -651,7 +657,7 @@ export function doc<AppModelType, DbModelType extends DocumentData>(
     ) {
       throw new FirestoreError(
         Code.INVALID_ARGUMENT,
-        'Expected first argument to collection() to be a CollectionReference, ' +
+        'Expected first argument to doc() to be a CollectionReference, ' +
           'a DocumentReference or FirebaseFirestore'
       );
     }

packages/firestore/test/lite/integration.test.ts

@@ -285,6 +285,12 @@ describe('doc', () => {
 
   it('validates path', () => {
     return withTestDb(db => {
+      expect(() => doc(undefined as any, 'coll/doc')).to.throw(
+        'Function doc() cannot be called with an empty first argument.'
+      );
+      expect(() => doc({} as any, 'coll/doc')).to.throw(
+        'Expected first argument to doc() to be a CollectionReference, a DocumentReference or FirebaseFirestore'
+      );
       expect(() => doc(db, 'coll')).to.throw(
         'Invalid document reference. Document references must have an even ' +
           'number of segments, but coll has 1.'