diff --git a/lib/types.nix b/lib/types.nix
index a7f9bf1946e..27b653ebb6c 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -103,7 +103,8 @@ rec {
     path = mkOptionType {
       name = "path";
       # Hacky: there is no ‘isPath’ primop.
-      check = x: builtins.substring 0 1 (toString x) == "/";
+      # need to check isString first because otherwise toString throws an error.
+      check = x: builtins.isString x && builtins.substring 0 1 (toString x) == "/";
       merge = mergeOneOption;
     };
 
diff --git a/nixos/modules/config/system-path.nix b/nixos/modules/config/system-path.nix
index 62390f452ba..d7979593f40 100644
--- a/nixos/modules/config/system-path.nix
+++ b/nixos/modules/config/system-path.nix
@@ -57,7 +57,7 @@ in
     environment = {
 
       systemPackages = mkOption {
-        type = types.listOf types.path;
+        type = types.listOf types.package;
         default = [];
         example = literalExample "[ pkgs.firefox pkgs.thunderbird ]";
         description = ''