diff --git a/scrapy/core/downloader/handlers/s3.py b/scrapy/core/downloader/handlers/s3.py
index dd7bce2be..d8bbdd326 100644
--- a/scrapy/core/downloader/handlers/s3.py
+++ b/scrapy/core/downloader/handlers/s3.py
@@ -82,7 +82,7 @@ class S3DownloadHandler(object):
             awsrequest = botocore.awsrequest.AWSRequest(
                 method=request.method,
                 url='%s://s3.amazonaws.com/%s%s' % (scheme, bucket, path),
-                headers=request.headers.to_native_string_dict(),
+                headers=request.headers.to_unicode_dict(),
                 data=request.body)
             self._signer.add_auth(awsrequest)
             request = request.replace(
diff --git a/scrapy/http/headers.py b/scrapy/http/headers.py
index d0c4cd0fb..62507eb19 100644
--- a/scrapy/http/headers.py
+++ b/scrapy/http/headers.py
@@ -79,7 +79,10 @@ class Headers(CaselessDict):
     def to_string(self):
         return headers_dict_to_raw(self)
 
-    def to_native_string_dict(self):
+    def to_unicode_dict(self):
+        """ Return headers as a CaselessDict with unicode keys
+        and unicode values. Multiple values are joined with ','.
+        """
         return CaselessDict(
             (to_unicode(key, encoding=self.encoding),
              to_unicode(b','.join(value), encoding=self.encoding))