diff --git a/third_party/nix/src/libstore/download.cc b/third_party/nix/src/libstore/download.cc
index e73181e73..fdc14b376 100644
--- a/third_party/nix/src/libstore/download.cc
+++ b/third_party/nix/src/libstore/download.cc
@@ -835,7 +835,8 @@ void Downloader::download(DownloadRequest&& request, Sink& sink) {
         state.wait(state->avail);
       }
 
-      chunk = state->data;
+      chunk = std::move(state->data);
+      state->data = std::string();
 
       state->request.notify_one();
     }