diff --git a/src/openai/_models.py b/src/openai/_models.py
index 9b1aeb30bf..6b6f8e9294 100644
--- a/src/openai/_models.py
+++ b/src/openai/_models.py
@@ -651,8 +651,8 @@ def _build_discriminated_union_meta(*, union: type, meta_annotations: tuple[Any,
                 # Note: if one variant defines an alias then they all should
                 discriminator_alias = field_info.alias
 
-                if field_info.annotation and is_literal_type(field_info.annotation):
-                    for entry in get_args(field_info.annotation):
+                if (annotation := getattr(field_info, 'annotation', None)) and is_literal_type(annotation):
+                    for entry in get_args(annotation):
                         if isinstance(entry, str):
                             mapping[entry] = variant