diff --git a/src/calibre/ebooks/metadata/fetch.py b/src/calibre/ebooks/metadata/fetch.py
index 50a6726a0f..8b82d3c972 100644
--- a/src/calibre/ebooks/metadata/fetch.py
+++ b/src/calibre/ebooks/metadata/fetch.py
@@ -273,10 +273,11 @@ def filter_metadata_results(item):
 
 def do_cover_check(item):
     item.has_cover = False
-    try:
-        item.has_cover = check_for_cover(item.isbn)
-    except:
-        pass # Cover not found
+    if item.isbn:
+        try:
+            item.has_cover = check_for_cover(item.isbn)
+        except:
+            pass # Cover not found
 
 def check_for_covers(items):
     threads = [Thread(target=do_cover_check, args=(item,)) for item in items]
diff --git a/src/calibre/ebooks/metadata/isbndb.py b/src/calibre/ebooks/metadata/isbndb.py
index d9f376c83d..356cc3f1b1 100644
--- a/src/calibre/ebooks/metadata/isbndb.py
+++ b/src/calibre/ebooks/metadata/isbndb.py
@@ -34,7 +34,8 @@ def fetch_metadata(url, max=100, timeout=5.):
             errmsg = soup.find('errormessage').string
             raise ISBNDBError('Error fetching metadata: '+errmsg)
         total_results = int(book_list['total_results'])
-        np = '&page_number=%s&'%(page_number+1)
+        page_number += 1
+        np = '&page_number=%s&'%page_number
         url = re.sub(r'\&page_number=\d+\&', np, url)
         books.extend(book_list.findAll('bookdata'))
         max -= 1