diff --git a/docs/changelog.rst b/docs/changelog.rst
index d93406db3..b877d4d19 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -4,6 +4,7 @@ Changelog
 
 Changes in 0.8.4
 ================
+- Handle dynamic fieldnames that look like digits (#434)
 - Added get_user_document and improve mongo_auth module (#423)
 - Added str representation of GridFSProxy (#424)
 - Update transform to handle docs erroneously passed to unset (#416)
diff --git a/mongoengine/base/document.py b/mongoengine/base/document.py
index b9c07cf50..cea2f09bf 100644
--- a/mongoengine/base/document.py
+++ b/mongoengine/base/document.py
@@ -762,7 +762,7 @@ def _lookup_field(cls, parts):
 
         for field_name in parts:
             # Handle ListField indexing:
-            if field_name.isdigit():
+            if field_name.isdigit() and hasattr(field, 'field'):
                 new_field = field.field
                 fields.append(field_name)
                 continue
diff --git a/tests/queryset/queryset.py b/tests/queryset/queryset.py
index 9c04c0bec..75708cabf 100644
--- a/tests/queryset/queryset.py
+++ b/tests/queryset/queryset.py
@@ -3299,6 +3299,13 @@ class Test(Document):
         Test.objects(test='foo').update_one(upsert=True, set__test='foo')
         self.assertTrue('_cls' in Test._collection.find_one())
 
+    def test_update_upsert_looks_like_a_digit(self):
+        class MyDoc(DynamicDocument):
+            pass
+        MyDoc.drop_collection()
+        self.assertEqual(1, MyDoc.objects.update_one(upsert=True, inc__47=1))
+        self.assertEqual(MyDoc.objects.get()['47'], 1)
+
     def test_read_preference(self):
         class Bar(Document):
             pass