diff --git a/pkgs/os-specific/linux/zfs/default.nix b/pkgs/os-specific/linux/zfs/default.nix
index 83309813239..ae51d3d4f98 100644
--- a/pkgs/os-specific/linux/zfs/default.nix
+++ b/pkgs/os-specific/linux/zfs/default.nix
@@ -27,7 +27,7 @@ let
         versionAtLeast kernel.version incompatibleKernelVersion then
        throw ''
          Linux v${kernel.version} is not yet supported by zfsonlinux v${version}.
-         ${stdenv.lib.optional (!isUnstable) "Try zfsUnstable or set the NixOS option boot.zfs.enableUnstable."}
+         ${stdenv.lib.optionalString (!isUnstable) "Try zfsUnstable or set the NixOS option boot.zfs.enableUnstable."}
        ''
     else stdenv.mkDerivation rec {
       name = "zfs-${configFile}-${version}${optionalString buildKernel "-${kernel.version}"}";