diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index b153c067f..320148789 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -819,10 +819,10 @@ def raise_errors_on_nested_writes(method_name, serializer, validated_data):
     #     ...
     #     address = serializer.CharField('profile.address')
     assert not any(
-        '.' in field.source and
-        (key in validated_data) and
-        isinstance(validated_data[key], (list, dict))
-        for key, field in serializer.fields.items()
+        len(field.source_attrs) > 1 and
+        (field.source_attrs[0] in validated_data) and
+        isinstance(validated_data[field.source_attrs[0]], (list, dict))
+        for field in serializer._writable_fields
     ), (
         'The `.{method_name}()` method does not support writable dotted-source '
         'fields by default.\nWrite an explicit `.{method_name}()` method for '