diff --git a/flask/testsuite/helpers.py b/flask/testsuite/helpers.py
index 7de70c0a..4bb43ac2 100644
--- a/flask/testsuite/helpers.py
+++ b/flask/testsuite/helpers.py
@@ -260,7 +260,7 @@ class SendfileTestCase(FlaskTestCase):
         app = flask.Flask(__name__)
         with catch_warnings() as captured:
             with app.test_request_context():
-                f = open(os.path.join(app.root_path, 'static/index.html'))
+                f = open(os.path.join(app.root_path, 'static/index.html'), mode='rb')
                 rv = flask.send_file(f)
                 rv.direct_passthrough = False
                 with app.open_resource('static/index.html') as f: