diff --git a/src/calibre/ebooks/metadata/sources/overdrive.py b/src/calibre/ebooks/metadata/sources/overdrive.py
index 0affbdd805..831dd473b3 100755
--- a/src/calibre/ebooks/metadata/sources/overdrive.py
+++ b/src/calibre/ebooks/metadata/sources/overdrive.py
@@ -23,7 +23,6 @@ from calibre.ebooks.chardet import xml_to_unicode
 from calibre.library.comments import sanitize_comments_html
 
 ovrdrv_data_cache = {}
-cover_url_cache = {}
 cache_lock = RLock()
 base_url = 'http://search.overdrive.com/'
 
@@ -96,8 +95,10 @@ class OverDrive(Source):
 
         ovrdrv_id = identifiers.get('overdrive', None)
         br = self.browser
-        referer = self.get_base_referer()+'ContentDetails-Cover.htm?ID='+ovrdrv_id
         req = mechanize.Request(cached_url)
+        if ovrdrv_id is not None:
+            referer = self.get_base_referer()+'ContentDetails-Cover.htm?ID='+ovrdrv_id
+            req.add_header('referer', referer)
         req.add_header('referer', referer)
         log('Downloading cover from:', cached_url)
         try:
@@ -240,7 +241,7 @@ class OverDrive(Source):
                 else:
                     creators = creators.split(', ')
                     # if an exact match in a preferred format occurs
-                    if (author and creators[0] == author[0]) and od_title == title and int(formatid) in [1, 50, 410, 900]:
+                    if (author and creators[0] == author[0]) and od_title == title and int(formatid) in [1, 50, 410, 900] and thumbimage:
                         return self.format_results(reserveid, od_title, subtitle, series, publisher,
                                 creators, thumbimage, worldcatlink, formatid)
                     else: