diff --git a/src/calibre/constants.py b/src/calibre/constants.py
index aefd0ea4ba..e44683c1ff 100644
--- a/src/calibre/constants.py
+++ b/src/calibre/constants.py
@@ -87,7 +87,7 @@ else:
         filesystem_encoding = 'utf-8'
 
 
-DEBUG = b'CALIBRE_DEBUG' in os.environ
+DEBUG = 'CALIBRE_DEBUG' in os.environ
 
 
 def debug():