diff --git a/src/calibre/ebooks/mobi/writer.py b/src/calibre/ebooks/mobi/writer.py
index 62d444ee95..d9f3a841ab 100644
--- a/src/calibre/ebooks/mobi/writer.py
+++ b/src/calibre/ebooks/mobi/writer.py
@@ -180,7 +180,6 @@ class Serializer(object):
         if not isinstance(elem.tag, basestring) \
            or namespace(elem.tag) not in nsrmap:
             return
-        hrefs = self.oeb.manifest.hrefs
         tag = prefixname(elem.tag, nsrmap)
         for attr in ('name', 'id'):
             if attr in elem.attrib:
@@ -203,7 +202,7 @@ class Serializer(object):
                         continue
                 elif attr == 'src':
                     href = item.abshref(val)
-                    if href in hrefs:
+                    if href in self.images:
                         index = self.images[href]
                         buffer.write('recindex="%05d"' % index)
                         continue
diff --git a/src/calibre/ebooks/oeb/transforms/trimmanifest.py b/src/calibre/ebooks/oeb/transforms/trimmanifest.py
index bd2c388245..b150a12831 100644
--- a/src/calibre/ebooks/oeb/transforms/trimmanifest.py
+++ b/src/calibre/ebooks/oeb/transforms/trimmanifest.py
@@ -41,8 +41,9 @@ class ManifestTrimmer(object):
         while unchecked:
             new = set()
             for item in unchecked:
-                if item.media_type in OEB_DOCS or \
-                   item.media_type[-4:] in ('/xml', '+xml'):
+                if (item.media_type in OEB_DOCS or 
+                    item.media_type[-4:] in ('/xml', '+xml')) and \
+                   item.data is not None:
                     hrefs = [sel(item.data) for sel in LINK_SELECTORS]
                     for href in chain(*hrefs):
                         href = item.abshref(href)