diff --git a/optional_plugins/robot/avocado_robot/__init__.py b/optional_plugins/robot/avocado_robot/__init__.py
index d2072e8b0c7f2393f6905d807f642974c1b34f64..f5842766e6f15b64ff925aa85de68e1d87b4b542 100644
--- a/optional_plugins/robot/avocado_robot/__init__.py
+++ b/optional_plugins/robot/avocado_robot/__init__.py
@@ -82,6 +82,10 @@ class RobotLoader(loader.TestLoader):
     def discover(self, url, which_tests=False):
         avocado_suite = []
         subtests_filter = None
+
+        if url is None:
+            return []
+
         if ':' in url:
             url, _subtests_filter = url.split(':', 1)
             subtests_filter = re.compile(_subtests_filter)