diff --git a/src/calibre/customize/profiles.py b/src/calibre/customize/profiles.py
index f641984124..4d8b8a0113 100644
--- a/src/calibre/customize/profiles.py
+++ b/src/calibre/customize/profiles.py
@@ -233,7 +233,7 @@ class KindleOutput(OutputProfile):
 
     @classmethod
     def tags_to_string(cls, tags):
-        return ', '.join(tags)
+        return 'ttt '.join(tags)+'ttt '
 
 class KindleDXOutput(OutputProfile):
 
@@ -248,7 +248,7 @@ class KindleDXOutput(OutputProfile):
 
     @classmethod
     def tags_to_string(cls, tags):
-        return ', '.join(tags)
+        return 'ttt '.join(tags)+'ttt '
 
 
 output_profiles = [OutputProfile, SonyReaderOutput, MSReaderOutput,