diff --git a/drf_spectacular/plumbing.py b/drf_spectacular/plumbing.py
index e3c1b1cc..497b4d53 100644
--- a/drf_spectacular/plumbing.py
+++ b/drf_spectacular/plumbing.py
@@ -1,5 +1,4 @@
 import collections
-import copy
 import functools
 import hashlib
 import inspect
@@ -534,7 +533,10 @@ def append_meta(schema: _SchemaType, meta: _SchemaType) -> _SchemaType:
 
         if schema_nullable or meta_nullable:
             if 'type' in schema:
-                schema['type'] = [schema['type'], 'null']
+                if isinstance(schema['type'], str):
+                    schema['type'] = [schema['type'], 'null']
+                else:
+                    schema['type'] = [*schema['type'], 'null']
             elif '$ref' in schema:
                 schema = {'oneOf': [schema, {'type': 'null'}]}
             elif len(schema) == 1 and 'oneOf' in schema:
diff --git a/tests/test_extend_schema.py b/tests/test_extend_schema.py
index 9a561b9d..f142eef2 100644
--- a/tests/test_extend_schema.py
+++ b/tests/test_extend_schema.py
@@ -257,6 +257,29 @@ def view_func(request, format=None):
     }
 
 
+@mock.patch('drf_spectacular.settings.spectacular_settings.OAS_VERSION', '3.1.0')
+def test_extend_schema_field_with_schema_as_oas_3_1(no_warnings):
+    @extend_schema_field({'type': ['string', 'integer']})
+    class CustomField(serializers.CharField):
+        pass
+
+    class XSerializer(serializers.Serializer):
+        field1 = CustomField(read_only=True, allow_null=True)
+        field2 = CustomField(read_only=True, allow_null=True)
+
+    @extend_schema(request=XSerializer, responses=XSerializer)
+    @api_view(['POST'])
+    def view_func(request, format=None):
+        pass  # pragma: no cover
+
+    schema = generate_schema('x', view_function=view_func)
+
+    assert schema['components']['schemas']['X']['properties'] == {
+        'field1': {'readOnly': True, 'type': ['string', 'integer', 'null']},
+        'field2': {'readOnly': True, 'type': ['string', 'integer', 'null']},
+    }
+
+
 def test_layered_extend_schema_on_view_and_method_with_meta(no_warnings):
     class XSerializer(serializers.Serializer):
         field = serializers.IntegerField()