diff --git a/homeassistant/components/camera/__init__.py b/homeassistant/components/camera/__init__.py
index d9ccb0490c6..4791a963048 100644
--- a/homeassistant/components/camera/__init__.py
+++ b/homeassistant/components/camera/__init__.py
@@ -696,10 +696,13 @@ async def async_handle_play_stream_service(
 
     # It is required to send a different payload for cast media players
     entity_ids = service_call.data[ATTR_MEDIA_PLAYER]
+    sources = entity_sources(hass)
     cast_entity_ids = [
         entity
-        for entity, source in entity_sources(hass).items()
-        if entity in entity_ids and source["domain"] == "cast"
+        for entity in entity_ids
+        # All entities should be in sources. This extra guard is to
+        # avoid people writing to the state machine and breaking it.
+        if entity in sources and sources[entity]["domain"] == "cast"
     ]
     other_entity_ids = list(set(entity_ids) - set(cast_entity_ids))