only try to download if we have a url

oml/item/models.py

@@ -843,15 +843,17 @@ def download_cover(id):
             url = i.meta.get('cover')
         else:
             url = None
+            cover = None
 
     logger.debug('download cover %s %s', id, url)
     ratio = None
-    try:
-        cover = ox.net.read_url(url)
-        ratio = get_ratio(cover)
-    except:
-        logger.debug('unable to read cover url %s', url)
-        cover = None
+    if url:
+        try:
+            cover = ox.net.read_url(url)
+            ratio = get_ratio(cover)
+        except:
+            logger.debug('unable to read cover url %s', url)
+            cover = None
     with db.session():
         i = Item.get(id, for_update=True)
         if i: