diff --git a/youtube_dl/extractor/zdf.py b/youtube_dl/extractor/zdf.py
index d41c4e712..ed385450c 100644
--- a/youtube_dl/extractor/zdf.py
+++ b/youtube_dl/extractor/zdf.py
@@ -75,9 +75,9 @@ def extract_from_xml_url(ie, video_id, xml_url):
         for node in fnode:
             thumbnail = {'url': node.text}
             if 'key' in node.attrib:
-                width_x_height = node.attrib['key']
-                thumbnail['width'] = int_or_none(width_x_height.split('x')[0])
-                thumbnail['height'] = int_or_none(width_x_height.split('x')[1])
+                if re.match("^[0-9]+x[0-9]+$", node.attrib['key']):
+                    thumbnail['width'] = int_or_none(node.attrib['key'].split('x')[0])
+                    thumbnail['height'] = int_or_none(node.attrib['key'].split('x')[1])
             thumbnails.append(thumbnail)
         return thumbnails