diff --git a/check.py b/check.py
index ee0b3de..0e36410 100755
--- a/check.py
+++ b/check.py
@@ -36,6 +36,7 @@ class AbsoluteDNSAdapter(HTTPAdapter):
 
         # Append a dot to the hostname if it's not already there.
         hostname = parsed_url.host
+        assert hostname
         if not hostname.endswith("."):
             hostname += "."