diff --git a/src/calibre/ebooks/mobi/utils.py b/src/calibre/ebooks/mobi/utils.py
index 0d55a8d4bd..ae8e583a1b 100644
--- a/src/calibre/ebooks/mobi/utils.py
+++ b/src/calibre/ebooks/mobi/utils.py
@@ -369,7 +369,7 @@ def count_set_bits(num):
 def to_base(num, base=32, min_num_digits=None):
     digits = string.digits + string.ascii_uppercase
     sign = 1 if num >= 0 else -1
-    if num == 0: return '0'
+    if num == 0: return ('0' if min_num_digits is None else '0'*min_num_digits)
     num *= sign
     ans = []
     while num:
diff --git a/src/calibre/ebooks/mobi/writer8/skeleton.py b/src/calibre/ebooks/mobi/writer8/skeleton.py
index 1bf280306f..fffaa1f14a 100644
--- a/src/calibre/ebooks/mobi/writer8/skeleton.py
+++ b/src/calibre/ebooks/mobi/writer8/skeleton.py
@@ -366,7 +366,7 @@ class Chunker(object):
         def to_placeholder(aid):
             pos, fid = aid_map[aid]
             pos, fid = to_base(pos, min_num_digits=4), to_href(fid)
-            return bytes(':'.join((pos, fid)))
+            return bytes(':off:'.join((pos, fid)))
 
         placeholder_map = {bytes(k):to_placeholder(v) for k, v in
                 self.placeholder_map.iteritems()}
@@ -376,12 +376,12 @@ class Chunker(object):
             raw = match.group()
             pl = match.group(1)
             try:
-                return raw[:-15] + placeholder_map[pl]
+                return raw[:-19] + placeholder_map[pl]
             except KeyError:
                 pass
             return raw
 
-        return re.sub(br'<[^>]+(kindle:pos:fid:0000:\d{10})', sub, text)
+        return re.sub(br'<[^>]+(kindle:pos:fid:0000:off:[0-9A-Za-z]{10})', sub, text)
 
     def dump(self, orig_dumps):
         import tempfile, shutil, os