diff --git a/src/nix-collect-garbage/nix-collect-garbage.cc b/src/nix-collect-garbage/nix-collect-garbage.cc
index 568a1fa7d..366a378a1 100644
--- a/src/nix-collect-garbage/nix-collect-garbage.cc
+++ b/src/nix-collect-garbage/nix-collect-garbage.cc
@@ -6,7 +6,7 @@
 
 using namespace nix;
 
-std::string gen = "";
+std::string gen = "old";
 bool dryRun = false;
 
 void runProgramSimple(Path program, const Strings & args)