diff --git a/pre_commit/commands/run.py b/pre_commit/commands/run.py
index 5d8dc005f..ad3d766ef 100644
--- a/pre_commit/commands/run.py
+++ b/pre_commit/commands/run.py
@@ -364,8 +364,9 @@ def run(
     # Expose prepare_commit_message_source / commit_object_name
     # as environment variables for the hooks
     if args.prepare_commit_message_source:
-        environ['PRE_COMMIT_COMMIT_MSG_SOURCE'] \
-            = args.prepare_commit_message_source
+        environ['PRE_COMMIT_COMMIT_MSG_SOURCE'] = (
+            args.prepare_commit_message_source
+        )
 
     if args.commit_object_name:
         environ['PRE_COMMIT_COMMIT_OBJECT_NAME'] = args.commit_object_name