diff --git a/src/calibre/ebooks/mobi/writer8/skeleton.py b/src/calibre/ebooks/mobi/writer8/skeleton.py
index 1fab295273..286083331c 100644
--- a/src/calibre/ebooks/mobi/writer8/skeleton.py
+++ b/src/calibre/ebooks/mobi/writer8/skeleton.py
@@ -16,7 +16,7 @@ from lxml import etree
 from calibre import my_unichr
 from calibre.ebooks.oeb.base import XHTML_NS, extract
 from calibre.ebooks.mobi.utils import to_base, PolyglotDict
-from polyglot.builtins import iteritems, unicode_type
+from polyglot.builtins import iteritems, unicode_type, as_bytes
 
 CHUNK_SIZE = 8192
 
@@ -397,7 +397,7 @@ class Chunker(object):
             pos, fid = to_base(pos, min_num_digits=4), to_href(fid)
             return ':off:'.join((pos, fid)).encode('utf-8')
 
-        placeholder_map = {k:to_placeholder(v) for k, v in
+        placeholder_map = {as_bytes(k):to_placeholder(v) for k, v in
                 iteritems(self.placeholder_map)}
 
         # Now update the links