diff --git a/src/aioweb/session.py b/src/aioweb/session.py
index 5ed4b37..a0dc4b3 100644
--- a/src/aioweb/session.py
+++ b/src/aioweb/session.py
@@ -291,7 +291,8 @@ class ExtendedSession:
         kwargs["proxy"] = self._get_proxy(url, kwargs.pop("proxies", None))
         debug = kwargs.pop("debug", False)
 
-        kwargs["headers"] = self._apply_overwrites(kwargs.get("headers"))
+        merged = {**self._default_headers, **(kwargs.get("headers") or {})}
+        kwargs["headers"] = self._apply_overwrites(merged)
         kwargs["headers"] = {str(k): str(v) for k, v in kwargs["headers"].items()}
 
         timeout = kwargs.get("timeout")