Pyramid on App Engine gets InvalidResponseError value: header values ​​must be str, received 'unicode'

I am using Pyramid 1.3 with the AppEngine 1.6.4 SDK on OS X 10.7.3. I am using Python 2.7 and has threadafe true in app.yaml.

@view_config(route_name='manager_swms', permission='manager', renderer='manager/swms.jinja2')
def manager_swms(request):
    """Generates blobstore url and passes users swms in swms table"""

    # generate url for any form upload that may occur
    upload_url = blobstore.create_upload_url('/upload_swm')

    user = get_current_user(request)
    swms = DBSession.query(SWMS).filter_by(owner_id=int(user.id)).all()

    return {
        "analytics_id": analytics_id,
        "user": get_current_user(request),
        "upload_url": upload_url,
        "swms": [(x.filename, x.blob_key) for x in swms]
    }

class BlobstoreUploadHandler(object):
    """Base class for creation blob upload handlers."""

    def __init__(self, *args, **kwargs):
        self.__uploads = None

    def get_uploads(self, field_name=None):
        """Get uploads sent to this handler.

        Args:
          field_name: Only select uploads that were sent as a specific field.

        Returns:
          A list of BlobInfo records corresponding to each upload.
          Empty list if there are no blob-info records for field_name.
        """
        if self.__uploads is None:
            self.__uploads = {}
            for key, value in self.request.params.items():
                if isinstance(value, cgi.FieldStorage):
                    if 'blob-key' in value.type_options:
                        self.__uploads.setdefault(key, []).append(
                            blobstore.parse_blob_info(value))

        if field_name:
            try:
                return list(self.__uploads[field_name])
            except KeyError:
                return []
        else:
            results = []
            for uploads in self.__uploads.itervalues():
                results += uploads
            return results

@view_config(route_name='upload_swm', permission='manager')
class UploadHandler(BlobstoreUploadHandler):
    ''' Handles redirects from Blobstore uploads. '''

    def __init__(self, request):
        self.request = request
        super(UploadHandler, self).__init__()

    def __call__(self):

        user = get_current_user(self.request)
        for blob_info in self.get_uploads('file'):

            new_swm = SWMS(
                owner_id = int(user.id),
                blob_key = str(blob_info.key()),
                filename = blob_info.filename,
                size = blob_info.size,
            )
            DBSession.add(new_swm)
        DBSession.flush()

        # redirect to swms page
        return HTTPFound(location='/manager/swms')

In the above code, manager_swms () creates a page that includes a form for uploading a file to Blobstore. The form works fine, and I see that the blob appears in the Blobstore when the form is used. The redirect from the POST of the BLOB store is then performed in / upload _swm, where I successfully take the BlobInfo data and put it into the SQL table. All this is good, and the last thing I want to do is redirect to the first page so that another file can be loaded if necessary, and I can show the list of downloaded files.

Pyramid, HTTPFound (location = '/manager/swms') [ URL- ], , :

ERROR    2012-03-29 22:56:38,170 wsgi.py:208] 
Traceback (most recent call last):
  File "/Users/tim/work/OHSPro/var/parts/google_appengine/google/appengine/runtime/wsgi.py", line 196, in Handle
    result = handler(self._environ, self._StartResponse)
  File "lib/dist/pyramid/router.py", line 195, in __call__
    foo = response(request.environ, start_response)
  File "lib/dist/pyramid/httpexceptions.py", line 291, in __call__
    foo = Response.__call__(self, environ, start_response)
  File "lib/dist/webob/response.py", line 922, in __call__
    start_response(self.status, headerlist)
  File "/Users/tim/work/OHSPro/var/parts/google_appengine/google/appengine/runtime/wsgi.py", line 150, in _StartResponse
    _GetTypeName(header[1]))
InvalidResponseError: header values must be str, got 'unicode'
INFO     2012-03-29 22:56:38,174 dev_appserver_blobstore.py:408] Upload handler returned 500
INFO     2012-03-29 22:56:38,193 dev_appserver.py:2884] "POST /_ah/upload/ahJkZXZ-cHJvdG8tc2NvaHNwcm9yGwsSFV9fQmxvYlVwbG9hZFNlc3Npb25fXxgTDA HTTP/1.1" 500 -

AppEngine unicode HTTP, AFAIK. pdb HTTPFound, :

ResponseHeaders([('Content-Type', 'text/html; charset=UTF-8'), ('Content-Length', '0'), ('Location', '/manager/swms')])

unicode?