diff --git a/src/calibre/__init__.py b/src/calibre/__init__.py
index eb428e9de4..7df3699234 100644
--- a/src/calibre/__init__.py
+++ b/src/calibre/__init__.py
@@ -474,7 +474,7 @@ def strftime(fmt, t=None):
 def my_unichr(num):
     try:
         return unichr(num)
-    except ValueError:
+    except (ValueError, OverflowError):
         return u'?'
 
 def entity_to_unicode(match, exceptions=[], encoding='cp1252',
diff --git a/src/calibre/ebooks/mobi/writer.py b/src/calibre/ebooks/mobi/writer.py
index c1cde59718..de26b58058 100644
--- a/src/calibre/ebooks/mobi/writer.py
+++ b/src/calibre/ebooks/mobi/writer.py
@@ -172,6 +172,9 @@ class Serializer(object):
         hrefs = self.oeb.manifest.hrefs
         buffer.write('<guide>')
         for ref in self.oeb.guide.values():
+            # The Kindle decides where to open a book based on the presence of
+            # an item in the guide that looks like
+            # <reference type="text" title="Start" href="chapter-one.xhtml"/>
             path = urldefrag(ref.href)[0]
             if path not in hrefs or hrefs[path].media_type not in OEB_DOCS:
                 continue