diff --git a/src/main/java/org/kohsuke/github/GHNotificationStream.java b/src/main/java/org/kohsuke/github/GHNotificationStream.java
index 7e62d7b6c6..f769907125 100644
--- a/src/main/java/org/kohsuke/github/GHNotificationStream.java
+++ b/src/main/java/org/kohsuke/github/GHNotificationStream.java
@@ -187,7 +187,9 @@ GHThread fetch() {
                             Thread.sleep(waitTime);
                         }
 
-                        req.setHeader("If-Modified-Since", lastModified);
+                        if (lastModified != null) {
+                            req.setHeader("If-Modified-Since", lastModified);
+                        }
 
                         Requester requester = req.withUrlPath(apiUrl);
                         GitHubResponse<GHThread[]> response = ((GitHubPageContentsIterable<GHThread>) requester