diff --git a/httpie/sessions.py b/httpie/sessions.py
index e61a8e3e..32254bfb 100644
--- a/httpie/sessions.py
+++ b/httpie/sessions.py
@@ -101,6 +101,10 @@ class Session(BaseConfigDict):
 
         """
         for name, value in request_headers.items():
+
+            if value is None:
+                continue  # Ignore explicitely unset headers
+
             value = value.decode('utf8')
             if name == 'User-Agent' and value.startswith('HTTPie/'):
                 continue
diff --git a/tests/test_sessions.py b/tests/test_sessions.py
index 966d87b4..f5683774 100644
--- a/tests/test_sessions.py
+++ b/tests/test_sessions.py
@@ -2,6 +2,7 @@
 import os
 import shutil
 import sys
+from tempfile import gettempdir
 
 import pytest
 
@@ -174,3 +175,14 @@ class TestSession(SessionTestBase):
         r2 = http('--session=test', httpbin.url + '/headers', env=self.env())
         assert HTTP_OK in r2
         assert r2.json['headers']['User-Agent'] == 'custom'
+
+    def test_download_in_session(self, httpbin):
+        # https://github.com/jkbrzt/httpie/issues/412
+        self.start_session(httpbin)
+        cwd = os.getcwd()
+        try:
+            os.chdir(gettempdir())
+            http('--session=test', '--download',
+                 httpbin.url + '/get', env=self.env())
+        finally:
+            os.chdir(cwd)