diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
index 10cd2470a..940cf3022 100644
--- a/src/libexpr/primops/fetchGit.cc
+++ b/src/libexpr/primops/fetchGit.cc
@@ -46,6 +46,8 @@ GitInfo exportGit(ref<Store> store, std::string uri,
             /* This is an unclean working tree. So copy all tracked
                files. */
 
+            warn("Git tree '%s' is dirty", uri);
+
             GitInfo gitInfo;
             gitInfo.ref = "HEAD";