diff --git a/homeassistant/components/google_assistant/trait.py b/homeassistant/components/google_assistant/trait.py
index 7d6e79a8237..6b5530ab2ce 100644
--- a/homeassistant/components/google_assistant/trait.py
+++ b/homeassistant/components/google_assistant/trait.py
@@ -185,6 +185,8 @@ class BrightnessTrait(_Trait):
             brightness = self.state.attributes.get(light.ATTR_BRIGHTNESS)
             if brightness is not None:
                 response["brightness"] = int(100 * (brightness / 255))
+            else:
+                response["brightness"] = 0
 
         return response
 
diff --git a/tests/components/google_assistant/test_smart_home.py b/tests/components/google_assistant/test_smart_home.py
index 2f7fdb8e131..3c3801b35c6 100644
--- a/tests/components/google_assistant/test_smart_home.py
+++ b/tests/components/google_assistant/test_smart_home.py
@@ -226,7 +226,7 @@ async def test_query_message(hass):
         "payload": {
             "devices": {
                 "light.non_existing": {"online": False},
-                "light.demo_light": {"on": False, "online": True},
+                "light.demo_light": {"on": False, "online": True, "brightness": 0},
                 "light.another_light": {
                     "on": True,
                     "online": True,