diff --git a/src/calibre/ebooks/mobi/debug/headers.py b/src/calibre/ebooks/mobi/debug/headers.py
index b367be2e4a..034c714d31 100644
--- a/src/calibre/ebooks/mobi/debug/headers.py
+++ b/src/calibre/ebooks/mobi/debug/headers.py
@@ -205,7 +205,10 @@ class EXTHHeader(object):
 
     @property
     def kf8_header_index(self):
-        return self.get(121, None)
+        ans = self.get(121, None)
+        if ans == NULL_INDEX:
+            ans = None
+        return ans
 
     def __str__(self):
         ans = ['*'*20 + ' EXTH Header '+ '*'*20]
@@ -467,9 +470,15 @@ class MOBIFile(object):
         if mh.file_version >= 8:
             self.kf8_type = 'standalone'
         elif mh.has_exth and mh.exth.kf8_header_index is not None:
-            self.kf8_type = 'joint'
             kf8i = mh.exth.kf8_header_index
-            mh8 = MOBIHeader(self.records[kf8i], kf8i)
+            try:
+                rec = self.records[kf8i-1]
+            except IndexError:
+                pass
+            else:
+                if rec.raw == b'BOUNDARY':
+                    self.kf8_type = 'joint'
+                    mh8 = MOBIHeader(self.records[kf8i], kf8i)
         self.mobi8_header = mh8
 
         if 'huff' in self.mobi_header.compression.lower():
diff --git a/src/calibre/ebooks/mobi/reader/headers.py b/src/calibre/ebooks/mobi/reader/headers.py
index 06d349d5de..571817f230 100644
--- a/src/calibre/ebooks/mobi/reader/headers.py
+++ b/src/calibre/ebooks/mobi/reader/headers.py
@@ -75,6 +75,8 @@ class EXTHHeader(object): # {{{
                 self.mi.author_sort = au.strip()
         elif idx == 101:
             self.mi.publisher = content.decode(codec, 'ignore').strip()
+            if self.mi.publisher in {'Unknown', _('Unknown')}:
+                self.mi.publisher = None
         elif idx == 103:
             self.mi.comments  = content.decode(codec, 'ignore')
         elif idx == 104:
@@ -98,6 +100,8 @@ class EXTHHeader(object): # {{{
             self.start_offset, = struct.unpack(b'>L', content)
         elif idx == 121:
             self.kf8_header, = struct.unpack(b'>L', content)
+            if self.kf8_header == NULL_INDEX:
+                self.kf8_header = None
         #else:
         #    print 'unhandled metadata record', idx, repr(content)
 # }}}