diff --git a/lib/types.nix b/lib/types.nix
index 7276f9af9fe..b833417e73d 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -193,9 +193,9 @@ rec {
 
     nullOr = elemType: mkOptionType {
       name = "null or ${elemType.name}";
-      check = x: builtins.isNull x || elemType.check x;
+      check = x: x == null || elemType.check x;
       merge = loc: defs:
-        let nrNulls = count (def: isNull def.value) defs; in
+        let nrNulls = count (def: def.value == null) defs; in
         if nrNulls == length defs then null
         else if nrNulls != 0 then
           throw "The option `${showOption loc}' is defined both null and not null, in ${showFiles (getFiles defs)}."
@@ -230,11 +230,18 @@ rec {
         substSubModules = m: submodule m;
       };
 
-    enum = values: mkOptionType {
-      name = "one of ${concatStringsSep ", " values}";
-      check = flip elem values;
-      merge = mergeOneOption;
-    };
+    enum = values:
+      let
+        show = v:
+               if builtins.isString v then ''"${v}"''
+          else if builtins.isInt v then builtins.toString v
+          else ''<${builtins.typeOf v}>'';
+      in
+      mkOptionType {
+        name = "one of ${concatMapStringsSep ", " show values}";
+        check = flip elem values;
+        merge = mergeOneOption;
+      };
 
     either = t1: t2: mkOptionType {
       name = "${t1.name} or ${t2.name}";