diff --git a/homeassistant/components/mqtt/fan.py b/homeassistant/components/mqtt/fan.py
index 5d69fb87b91..e5cebd43714 100644
--- a/homeassistant/components/mqtt/fan.py
+++ b/homeassistant/components/mqtt/fan.py
@@ -365,7 +365,7 @@ class MqttFan(MqttEntity, FanEntity):
         elif speed == SPEED_OFF:
             mqtt_payload = self._payload["SPEED_OFF"]
         else:
-            mqtt_payload = speed
+            raise ValueError(f"{speed} is not a valid fan speed")
 
         mqtt.async_publish(
             self.hass,
diff --git a/tests/components/mqtt/test_fan.py b/tests/components/mqtt/test_fan.py
index d52b67fd3ed..045b8fdaf0e 100644
--- a/tests/components/mqtt/test_fan.py
+++ b/tests/components/mqtt/test_fan.py
@@ -435,14 +435,8 @@ async def test_sending_mqtt_commands_and_explicit_optimistic(hass, mqtt_mock):
     assert state.state is STATE_OFF
     assert state.attributes.get(ATTR_ASSUMED_STATE)
 
-    await common.async_set_speed(hass, "fan.test", "cUsToM")
-    mqtt_mock.async_publish.assert_called_once_with(
-        "speed-command-topic", "cUsToM", 0, False
-    )
-    mqtt_mock.async_publish.reset_mock()
-    state = hass.states.get("fan.test")
-    assert state.state is STATE_OFF
-    assert state.attributes.get(ATTR_ASSUMED_STATE)
+    with pytest.raises(ValueError):
+        await common.async_set_speed(hass, "fan.test", "cUsToM")
 
 
 async def test_attributes(hass, mqtt_mock):
@@ -522,12 +516,8 @@ async def test_attributes(hass, mqtt_mock):
     assert state.attributes.get(fan.ATTR_SPEED) == "off"
     assert state.attributes.get(fan.ATTR_OSCILLATING) is False
 
-    await common.async_set_speed(hass, "fan.test", "cUsToM")
-    state = hass.states.get("fan.test")
-    assert state.state is STATE_OFF
-    assert state.attributes.get(ATTR_ASSUMED_STATE)
-    assert state.attributes.get(fan.ATTR_SPEED) == "cUsToM"
-    assert state.attributes.get(fan.ATTR_OSCILLATING) is False
+    with pytest.raises(ValueError):
+        await common.async_set_speed(hass, "fan.test", "cUsToM")
 
 
 async def test_custom_speed_list(hass, mqtt_mock):