diff --git a/graphene_mongo/converter.py b/graphene_mongo/converter.py
index f4cce42..5a6f37e 100644
--- a/graphene_mongo/converter.py
+++ b/graphene_mongo/converter.py
@@ -350,10 +350,20 @@ def convert_field_to_union(field, registry=None, executor: ExecutorEnum = Execut
     if len(_types) == 0:
         return None
 
-    name = (
-        to_camel_case("{}_{}".format(field._owner_document.__name__, field.db_field)) + "UnionType"
-        if ExecutorEnum.SYNC
-        else "AsyncUnionType"
+    field_name = field.db_field
+    if field_name is None:
+        # Get db_field name from parent mongo_field
+        for db_field_name, _mongo_parent_field in field.owner_document._fields.items():
+            if hasattr(_mongo_parent_field, "field") and _mongo_parent_field.field == field:
+                field_name = db_field_name
+                break
+
+    name = to_camel_case(
+        "{}_{}_{}".format(
+            field._owner_document.__name__,
+            field_name,
+            "union_type" if executor == ExecutorEnum.SYNC else "async_union_type",
+        )
     )
     Meta = type("Meta", (object,), {"types": tuple(_types)})
     _union = type(name, (graphene.Union,), {"Meta": Meta})
diff --git a/pyproject.toml b/pyproject.toml
index 0bc3d0a..d2cebef 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,7 +1,7 @@
 [tool.poetry]
 name = "graphene-mongo"
 packages = [{ include = "graphene_mongo" }]
-version = "0.4.0"
+version = "0.4.1"
 description = "Graphene Mongoengine integration"
 authors = [
     "Abaw Chen <abaw.chen@gmail.com>",