diff --git a/httpie/client.py b/httpie/client.py
index fb81667f..c0f7a50d 100644
--- a/httpie/client.py
+++ b/httpie/client.py
@@ -19,6 +19,8 @@ from httpie.utils import repr_dict
 try:
     # noinspection PyPackageRequirements
     import urllib3
+
+
     # <https://urllib3.readthedocs.io/en/latest/security.html>
     urllib3.disable_warnings()
 except (ImportError, AttributeError):
@@ -181,7 +183,10 @@ def make_default_headers(args: argparse.Namespace) -> RequestHeadersDict:
     return default_headers
 
 
-def make_requests_kwargs(args: argparse.Namespace, base_headers=None) -> dict:
+def make_requests_kwargs(
+    args: argparse.Namespace,
+    base_headers: RequestHeadersDict = None
+) -> dict:
     """
     Translate our `args` into `requests.request` keyword arguments.
 
diff --git a/httpie/sessions.py b/httpie/sessions.py
index 69289647..4f80cc30 100644
--- a/httpie/sessions.py
+++ b/httpie/sessions.py
@@ -12,6 +12,7 @@ from requests.auth import AuthBase
 from requests.cookies import RequestsCookieJar, create_cookie
 import requests
 
+from httpie.cli.dicts import RequestHeadersDict
 from httpie.config import BaseConfigDict, DEFAULT_CONFIG_DIR
 from httpie.plugins import plugin_manager
 
@@ -101,14 +102,13 @@ class Session(BaseConfigDict):
     def _get_path(self) -> Path:
         return self._path
 
-    def update_headers(self, request_headers: dict):
+    def update_headers(self, request_headers: RequestHeadersDict):
         """
         Update the session headers with the request ones while ignoring
         certain name prefixes.
 
-        :type request_headers: dict
-
         """
+        headers = self.headers
         for name, value in request_headers.items():
 
             if value is None:
@@ -122,11 +122,13 @@ class Session(BaseConfigDict):
                 if name.lower().startswith(prefix.lower()):
                     break
             else:
-                self['headers'][name] = value
+                headers[name] = value
+
+        self['headers'] = dict(headers)
 
     @property
-    def headers(self) -> dict:
-        return self['headers']
+    def headers(self) -> RequestHeadersDict:
+        return RequestHeadersDict(self['headers'])
 
     @property
     def cookies(self) -> RequestsCookieJar: