Fix blueprint route for ""

flask/blueprints.py

@@ -66,7 +66,10 @@ class BlueprintSetupState(object):
         blueprint's name.
         """
         if self.url_prefix is not None:
-            rule = '/'.join((self.url_prefix, rule.lstrip('/')))
+            if rule:
+                rule = '/'.join((self.url_prefix, rule.lstrip('/')))
+            else:
+                rule = self.url_prefix
         options.setdefault('subdomain', self.subdomain)
         if endpoint is None:
             endpoint = _endpoint_from_view_func(view_func)

tests/test_blueprints.py

@@ -116,6 +116,7 @@ def test_blueprint_app_error_handling(app, client):
 
 
 @pytest.mark.parametrize(('prefix', 'rule', 'url'), (
+    ('/foo', '', '/foo'),
     ('/foo/', '/bar', '/foo/bar'),
     ('/foo/', 'bar', '/foo/bar'),
     ('/foo', '/bar', '/foo/bar'),