diff --git a/hosts/bagel-box/default.nix b/hosts/bagel-box/default.nix
index e09cb29..8d5510c 100644
--- a/hosts/bagel-box/default.nix
+++ b/hosts/bagel-box/default.nix
@@ -40,7 +40,7 @@
     hydra.enable = true;
     hydra.dbi = "dbi:Pg:dbname=hydra;user=hydra";
     # Takes 10 builders (0 → 9).
-    hydra.builders = lib.genList (i: "builder-${i}") 9;
+    hydra.builders = lib.genList (i: "builder-${builtins.toString i}") 9;
 
     ofborg.enable = true;
   };