diff --git a/zfs.nix b/zfs.nix
index 14a9270..1a3bae8 100644
--- a/zfs.nix
+++ b/zfs.nix
@@ -25,7 +25,7 @@ in
       mb = 4096;
     in
     [
-      "zfs.zfs_arc_max=${mb * 1000000}"
+      "zfs.zfs_arc_max=${builtins.toString (mb * 1000000)}"
     ];
 
   boot.supportedFilesystems = [ "zfs" ];