diff --git a/doc/changelog.txt b/doc/changelog.txt
index db373455..d21849cd 100644
--- a/doc/changelog.txt
+++ b/doc/changelog.txt
@@ -14,6 +14,7 @@ Changes:
 Fixes:
 - checking: Correct typos in the proxy handling code.
   Closes: GH bug #536
+- checking: Add to default HTTP client headers instead of replacing.
 - cmdline: Reactivate paging of help pages.
 
 
diff --git a/linkcheck/director/aggregator.py b/linkcheck/director/aggregator.py
index eb433a50..64abf983 100644
--- a/linkcheck/director/aggregator.py
+++ b/linkcheck/director/aggregator.py
@@ -44,9 +44,9 @@ def new_request_session(config, cookies):
     if cookies:
         session.cookies = cookies
     session.max_redirects = config["maxhttpredirects"]
-    session.headers = {
+    session.headers.update({
         "User-Agent": config["useragent"],
-    }
+    })
     if config["cookiefile"]:
         for cookie in cookies.from_file(config["cookiefile"]):
             session.cookies = requests.cookies.merge_cookies(session.cookies, cookie)