diff --git a/vcr/stubs/__init__.py b/vcr/stubs/__init__.py
index 6cc1bfa..d49e78b 100644
--- a/vcr/stubs/__init__.py
+++ b/vcr/stubs/__init__.py
@@ -55,9 +55,9 @@ def parse_headers(header_list):
 
 def serialize_headers(response):
     out = {}
-    for key in response.msg.keys():
+    for key, values in compat.get_headers(response):
         out.setdefault(key, [])
-        out[key].extend(response.msg.getheaders(key))
+        out[key].extend(values)
     return out
 
 
diff --git a/vcr/stubs/compat.py b/vcr/stubs/compat.py
index 3b383e5..6b65ca0 100644
--- a/vcr/stubs/compat.py
+++ b/vcr/stubs/compat.py
@@ -28,12 +28,11 @@ def get_header_items(message):
 
 
 def get_headers(response):
-    if six.PY3:
-        header_list = response.msg._headers
-        return [b': '.join((k.encode('utf-8'), v.encode('utf-8'))) + b'\r\n'
-                for k, v in header_list]
-    else:
-        return response.msg.headers
+    for key in response.msg.keys():
+        if six.PY3:
+            yield key, response.msg.get_all(key)
+        else:
+            yield key, response.msg.getheaders(key)
 
 
 def get_httpmessage(headers):