diff --git a/mockfirestore/query.py b/mockfirestore/query.py
index a8641d7..6cac8fa 100644
--- a/mockfirestore/query.py
+++ b/mockfirestore/query.py
@@ -71,7 +71,7 @@ def stream(self, transaction=None) -> Iterator[DocumentSnapshot]:
             for key, direction in reversed(self.orders):  # Process least significant first
                 doc_snapshots = sorted(
                     doc_snapshots,
-                    key=lambda doc: doc.to_dict()[key],
+                    key=lambda doc: doc.get(key),
                     reverse=direction == "DESCENDING",
                 )
         if self._start_at:
diff --git a/tests/test_collection_reference.py b/tests/test_collection_reference.py
index 9edc4c2..8dba919 100644
--- a/tests/test_collection_reference.py
+++ b/tests/test_collection_reference.py
@@ -547,3 +547,31 @@ def test_collection_useDocumentIdKwarg(self):
         }}
         doc = fs.collection('foo').document(document_id='first').get()
         self.assertEqual({'id': 1}, doc.to_dict())
+        
+    def test_collection_orderBy_with_nested_field(self):
+        fs = MockFirestore()
+        fs._data = {'foo': {
+            'first': {'user': {'name': 'Charlie', 'age': 30}},
+            'second': {'user': {'name': 'Alice', 'age': 25}},
+            'third': {'user': {'name': 'Bob', 'age': 35}}
+        }}
+        # Test ordering by nested field using dot notation
+        docs = list(fs.collection('foo').order_by('user.name').stream())
+        self.assertEqual(3, len(docs))
+        self.assertEqual('Alice', docs[0].get('user.name'))
+        self.assertEqual('Bob', docs[1].get('user.name'))
+        self.assertEqual('Charlie', docs[2].get('user.name'))
+        
+        # Test ordering by another nested field
+        docs = list(fs.collection('foo').order_by('user.age').stream())
+        self.assertEqual(3, len(docs))
+        self.assertEqual(25, docs[0].get('user.age'))
+        self.assertEqual(30, docs[1].get('user.age'))
+        self.assertEqual(35, docs[2].get('user.age'))
+        
+        # Test ordering in descending order
+        docs = list(fs.collection('foo').order_by('user.age', direction='DESCENDING').stream())
+        self.assertEqual(3, len(docs))
+        self.assertEqual(35, docs[0].get('user.age'))
+        self.assertEqual(30, docs[1].get('user.age'))
+        self.assertEqual(25, docs[2].get('user.age'))