diff --git a/netlib/http/headers.py b/netlib/http/headers.py
index 2caf8d519..6067ff5e5 100644
--- a/netlib/http/headers.py
+++ b/netlib/http/headers.py
@@ -76,7 +76,7 @@ class Headers(MultiDict):
         For use with the "Set-Cookie" header, see :py:meth:`get_all`.
     """
 
-    def __init__(self, fields=None, **headers):
+    def __init__(self, fields=(), **headers):
         """
         Args:
             fields: (optional) list of ``(name, value)`` header byte tuples,
diff --git a/netlib/http/request.py b/netlib/http/request.py
index 5a528bf21..a6e2a5efb 100644
--- a/netlib/http/request.py
+++ b/netlib/http/request.py
@@ -19,7 +19,7 @@ host_header_re = re.compile(r"^(?P<host>[^:]+|\[.+\])(?::(?P<port>\d+))?$")
 
 
 class RequestData(MessageData):
-    def __init__(self, first_line_format, method, scheme, host, port, path, http_version, headers=None, content=None,
+    def __init__(self, first_line_format, method, scheme, host, port, path, http_version, headers=(), content=None,
                  timestamp_start=None, timestamp_end=None):
         if not isinstance(headers, Headers):
             headers = Headers(headers)
diff --git a/netlib/http/response.py b/netlib/http/response.py
index 7d272e104..a6a5bf474 100644
--- a/netlib/http/response.py
+++ b/netlib/http/response.py
@@ -11,7 +11,7 @@ from .. import utils
 
 
 class ResponseData(MessageData):
-    def __init__(self, http_version, status_code, reason=None, headers=None, content=None,
+    def __init__(self, http_version, status_code, reason=None, headers=(), content=None,
                  timestamp_start=None, timestamp_end=None):
         if not isinstance(headers, Headers):
             headers = Headers(headers)
diff --git a/test/mitmproxy/test_flow_export.py b/test/mitmproxy/test_flow_export.py
index 0374ba7d8..af3713de3 100644
--- a/test/mitmproxy/test_flow_export.py
+++ b/test/mitmproxy/test_flow_export.py
@@ -22,7 +22,7 @@ def python_equals(testdata, text):
 
 req_get = lambda: netlib.tutils.treq(method='GET', content='', path=b"/path?a=foo&a=bar&b=baz")
 
-req_post = lambda: netlib.tutils.treq(method='POST', headers=None)
+req_post = lambda: netlib.tutils.treq(method='POST', headers=())
 
 req_patch = lambda: netlib.tutils.treq(method='PATCH', path=b"/path?query=param")
 
diff --git a/test/netlib/http/http2/test_connections.py b/test/netlib/http/http2/test_connections.py
index 9312576f0..139d4fc15 100644
--- a/test/netlib/http/http2/test_connections.py
+++ b/test/netlib/http/http2/test_connections.py
@@ -460,7 +460,7 @@ class TestAssembleRequest(object):
             b'',
             b'/',
             b"HTTP/2.0",
-            None,
+            (),
             None,
         ))
         assert len(bytes) == 1
@@ -475,7 +475,7 @@ class TestAssembleRequest(object):
             b'',
             b'/',
             b"HTTP/2.0",
-            None,
+            (),
             None,
         )
         req.stream_id = 0x42
diff --git a/test/netlib/http/test_request.py b/test/netlib/http/test_request.py
index fae7aefe5..c03db3393 100644
--- a/test/netlib/http/test_request.py
+++ b/test/netlib/http/test_request.py
@@ -13,7 +13,7 @@ class TestRequestData(object):
         with raises(ValueError):
             treq(headers="foobar")
 
-        assert isinstance(treq(headers=None).headers, Headers)
+        assert isinstance(treq(headers=()).headers, Headers)
 
 
 class TestRequestCore(object):
diff --git a/test/netlib/http/test_response.py b/test/netlib/http/test_response.py
index 63c976a26..5eac2a15f 100644
--- a/test/netlib/http/test_response.py
+++ b/test/netlib/http/test_response.py
@@ -15,7 +15,7 @@ class TestResponseData(object):
         with raises(ValueError):
             tresp(headers="foobar")
 
-        assert isinstance(tresp(headers=None).headers, Headers)
+        assert isinstance(tresp(headers=()).headers, Headers)
 
 
 class TestResponseCore(object):