diff --git a/src/calibre/utils/cleantext.py b/src/calibre/utils/cleantext.py
index f421fdcba2..89101a6219 100644
--- a/src/calibre/utils/cleantext.py
+++ b/src/calibre/utils/cleantext.py
@@ -12,6 +12,8 @@ def clean_ascii_chars(txt, charlist=None):
     Remove ASCII control chars: 0 to 8 and 11, 12, 14-31 by default
     This is all control chars except \\t,\\n and \\r
     '''
+    if not txt:
+        return ''
     global _ascii_pat
     if _ascii_pat is None:
         chars = list(range(8)) + [0x0B, 0x0C] + list(range(0x0E, 0x1F))