diff --git a/pkgs/pkgs-lib/formats.nix b/pkgs/pkgs-lib/formats.nix
index ef7d104f20e4..a184b0cbce53 100644
--- a/pkgs/pkgs-lib/formats.nix
+++ b/pkgs/pkgs-lib/formats.nix
@@ -1017,7 +1017,13 @@ optionalAttrs allowAliases aliases
     }:
     if format == "badgerfish" then
       {
-        type = serializableValueWith { typeName = "XML"; };
+        type =
+          attrsOf (serializableValueWith {
+            typeName = "XML";
+          })
+          // {
+            description = "XML value";
+          };
 
         generate =
           name: value:
diff --git a/pkgs/pkgs-lib/tests/formats.nix b/pkgs/pkgs-lib/tests/formats.nix
index ee6d817287c2..242fd64bc70a 100644
--- a/pkgs/pkgs-lib/tests/formats.nix
+++ b/pkgs/pkgs-lib/tests/formats.nix
@@ -142,6 +142,14 @@ runBuildTests {
     '';
   };
 
+  jsonNull = shouldPass {
+    format = formats.json { };
+    input = null;
+    expected = ''
+      null
+    '';
+  };
+
   yaml_1_1Atoms = shouldPass {
     format = formats.yaml_1_1 { };
     input = {
@@ -176,6 +184,15 @@ runBuildTests {
     '';
   };
 
+  yaml_1_1Null = shouldPass {
+    format = formats.yaml_1_1 { };
+    input = null;
+    expected = ''
+      null
+      ...
+    '';
+  };
+
   yaml_1_2Atoms = shouldPass {
     format = formats.yaml_1_2 { };
     input = {
@@ -210,6 +227,16 @@ runBuildTests {
     '';
   };
 
+  yaml_1_2Null = shouldPass {
+    format = formats.yaml_1_2 { };
+    input = null;
+    # nixfmt insists on removing indentation, so force it with ${"  "}
+    expected = ''
+
+      ${"  "}null
+    '';
+  };
+
   iniAtoms = shouldPass {
     format = formats.ini { };
     input = {
@@ -879,6 +906,14 @@ runBuildTests {
     '';
   };
 
+  cdnNull = shouldPass {
+    format = formats.cdn { };
+    input = null;
+    expected = ''
+      null: null
+    '';
+  };
+
   # This test is responsible for
   #   1. testing type coercions
   #   2. providing a more readable example test
@@ -989,6 +1024,14 @@ runBuildTests {
     '';
   };
 
+  luaNull = shouldPass {
+    format = formats.lua { };
+    input = null;
+    expected = ''
+      return nil
+    '';
+  };
+
   nixConfAtoms = shouldPass {
     format = formats.nixConf {
       package = pkgs.nix;
@@ -1014,6 +1057,15 @@ runBuildTests {
     '';
   };
 
+  nixConfNull = shouldFail {
+    format = formats.nixConf {
+      package = pkgs.nix;
+      version = pkgs.nix.version;
+      extraOptions = "ignore-try = false";
+    };
+    input = null;
+  };
+
   phpAtoms = shouldPass rec {
     format = formats.php { finalVariable = "config"; };
     input = {
@@ -1043,6 +1095,16 @@ runBuildTests {
     '';
   };
 
+  phpNull = shouldPass {
+    format = formats.php { finalVariable = "config"; };
+    input = null;
+    expected = ''
+      <?php
+      declare(strict_types=1);
+      $config = null;
+    '';
+  };
+
   pythonVars = shouldPass (
     let
       format = formats.pythonVars { };
@@ -1100,6 +1162,11 @@ runBuildTests {
     }
   );
 
+  pythonVarsNull = shouldFail {
+    format = formats.pythonVars { };
+    input = null;
+  };
+
   phpReturn = shouldPass {
     format = formats.php { };
     input = {
@@ -1145,6 +1212,11 @@ runBuildTests {
     '';
   };
 
+  xmlNull = shouldFail {
+    format = formats.xml { };
+    input = null;
+  };
+
   PlistGenerate = shouldPass {
     format = formats.plist { };
     input = {
@@ -1220,6 +1292,17 @@ runBuildTests {
       </plist>'';
   };
 
+  PlistNull = shouldPass {
+    format = formats.plist { };
+    input = null;
+    expected = ''
+      <?xml version="1.0" encoding="UTF-8"?>
+      <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+      <plist version="1.0">
+
+      </plist>'';
+  };
+
   hcl1Atoms = shouldPass {
     format = formats.hcl1 { };
     input = {