diff --git a/src/black/__init__.py b/src/black/__init__.py
index c1907d9..e09f08d 100644
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -883,7 +883,7 @@ def format_file_in_place(
         dst_name = f"{src}\t{now} +0000"
         diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
 
-        if write_back == write_back.COLOR_DIFF:
+        if write_back == WriteBack.COLOR_DIFF:
             diff_contents = color_diff(diff_contents)
 
         with lock or nullcontext():