diff --git a/drf_spectacular/plumbing.py b/drf_spectacular/plumbing.py
index ae47f236..c5a6a03b 100644
--- a/drf_spectacular/plumbing.py
+++ b/drf_spectacular/plumbing.py
@@ -533,6 +533,8 @@ def append_meta(schema: _SchemaType, meta: _SchemaType) -> _SchemaType:
                 schema['type'] = [schema['type'], 'null']
             elif '$ref' in schema:
                 schema = {'oneOf': [schema, {'type': 'null'}]}
+            elif len(schema) == 1 and 'oneOf' in schema:
+                schema['oneOf'].append({'type': 'null'})
             else:
                 assert False, 'Invalid nullable case'  # pragma: no cover