diff --git a/src/calibre/ebooks/mobi/writer.py b/src/calibre/ebooks/mobi/writer.py
index 1b19c13872..35f5835e11 100644
--- a/src/calibre/ebooks/mobi/writer.py
+++ b/src/calibre/ebooks/mobi/writer.py
@@ -478,15 +478,15 @@ class MobiWriter(object):
         # Variables for trailing byte sequence
         tbsType = 0x00
         tbSequence = ""
-        
+
         # Generate TBS for type 0x002 - mobi_book
         if self._initialIndexRecordFound == False :
-            
+
             # Is there any indexed content yet?
             if self._HTMLRecords[nrecords].currentSectionNodeCount == -1 :
                 # No indexing data - write vwi length of 1 only
                 tbSequence = decint(len(tbSequence) + 1, DECINT_FORWARD)
-                
+
             else :
                 # First indexed HTML record is a special case
                 # One or more nodes
@@ -495,7 +495,7 @@ class MobiWriter(object):
                     tbsType = 2
                 else :
                     tbsType = 6
-    
+
                 tbSequence = decint(tbsType, DECINT_FORWARD)
                 tbSequence += decint(0x00, DECINT_FORWARD)
                 # Don't write a nodecount for opening type 2 record
@@ -1104,7 +1104,7 @@ class MobiWriter(object):
                 exth.write(data)
                 nrecs += 1
         if oeb.metadata.cover:
-            id = str(oeb.metadata.cover[0])
+            id = unicode(oeb.metadata.cover[0])
             item = oeb.manifest.ids[id]
             href = item.href
             index = self._images[href] - 1
diff --git a/src/calibre/ebooks/oeb/base.py b/src/calibre/ebooks/oeb/base.py
index 67a8f04cc8..4410b762b6 100644
--- a/src/calibre/ebooks/oeb/base.py
+++ b/src/calibre/ebooks/oeb/base.py
@@ -725,7 +725,7 @@ class Manifest(object):
             self._data = data
 
         def __repr__(self):
-            return 'Item(id=%r, href=%r, media_type=%r)' \
+            return u'Item(id=%r, href=%r, media_type=%r)' \
                 % (self.id, self.href, self.media_type)
 
         def _parse_xhtml(self, data):
diff --git a/src/calibre/ebooks/oeb/transforms/rasterize.py b/src/calibre/ebooks/oeb/transforms/rasterize.py
index 2d86fe63b5..1549838bfe 100644
--- a/src/calibre/ebooks/oeb/transforms/rasterize.py
+++ b/src/calibre/ebooks/oeb/transforms/rasterize.py
@@ -40,7 +40,7 @@ class SVGRasterizer(object):
     @classmethod
     def generate(cls, opts):
         return cls()
-    
+
     def __call__(self, oeb, context):
         oeb.logger.info('Rasterizing SVG images...')
         self.oeb = oeb
@@ -98,7 +98,7 @@ class SVGRasterizer(object):
             data = "data:%s;base64,%s" % (linkee.media_type, data)
             elem.attrib[XLINK('href')] = data
         return svg
-            
+
     def rasterize_spine(self):
         for item in self.oeb.spine:
             html = item.data
@@ -182,12 +182,12 @@ class SVGRasterizer(object):
             elem.text = None
         for child in elem:
             elem.remove(child)
-    
+
     def rasterize_cover(self):
         covers = self.oeb.metadata.cover
         if not covers:
             return
-        cover = self.oeb.manifest.ids[str(covers[0])]
+        cover = self.oeb.manifest.ids[unicode(covers[0])]
         if not cover.media_type == SVG_MIME:
             return
         width = (self.profile.width / 72) * self.profile.dpi