diff --git a/src/drf_yasg/inspectors/base.py b/src/drf_yasg/inspectors/base.py
index db2b19c1..9d9029ae 100644
--- a/src/drf_yasg/inspectors/base.py
+++ b/src/drf_yasg/inspectors/base.py
@@ -281,7 +281,8 @@ def SwaggerType(existing_object=None, use_field_title=True, **instance_kwargs):
                 instance_kwargs.setdefault('title', title)
             if description is not None:
                 instance_kwargs.setdefault('description', description)
-            if field.allow_null:
+
+            if getattr(field, 'allow_null', None):
                 instance_kwargs['x_nullable'] = True
 
             instance_kwargs.update(kwargs)