diff --git a/starlette/responses.py b/starlette/responses.py
index 7d5afbae..e843a1e4 100644
--- a/starlette/responses.py
+++ b/starlette/responses.py
@@ -156,6 +156,7 @@ class UJSONResponse(JSONResponse):
     media_type = "application/json"
 
     def render(self, content: typing.Any) -> bytes:
+        assert ujson is not None, "ujson must be installed to use UJSONResponse"
         return ujson.dumps(content, ensure_ascii=False).encode("utf-8")