diff --git a/memorious/logic/http.py b/memorious/logic/http.py
index 511b4ca..abbc210 100644
--- a/memorious/logic/http.py
+++ b/memorious/logic/http.py
@@ -253,6 +253,9 @@ def last_modified(self):
         if last_modified_header is not None:
             # Tue, 15 Nov 1994 12:45:26 GMT
             last_modified = parse_date(last_modified_header)
+            if last_modified is None:
+                return None
+
             if last_modified < now + timedelta(seconds=30):
                 return last_modified.strftime("%Y-%m-%dT%H:%M:%S%z")
         return None
diff --git a/setup.py b/setup.py
index 51b71f8..8d577f4 100644
--- a/setup.py
+++ b/setup.py
@@ -2,7 +2,7 @@
 
 setup(
     name="memorious",
-    version="2.5.0",
+    version="2.5.1",
     description="A minimalistic, recursive web crawling library for Python.",
     long_description="",
     classifiers=[