diff --git a/spacy/tokens/_serialize.py b/spacy/tokens/_serialize.py
index 57bc98f4b..41f524839 100644
--- a/spacy/tokens/_serialize.py
+++ b/spacy/tokens/_serialize.py
@@ -116,7 +116,7 @@ def merge_boxes(boxes):
     if merged is not None:
         return merged.to_bytes()
     else:
-        return b''
+        return b""
 
 
 def pickle_box(box):