diff --git a/nixos/modules/misc/nixpkgs/test-nixpkgs-config-allow-unfree.nix b/nixos/modules/misc/nixpkgs/test-nixpkgs-config-allow-unfree.nix
new file mode 100644
index 000000000000..201a43314fbf
--- /dev/null
+++ b/nixos/modules/misc/nixpkgs/test-nixpkgs-config-allow-unfree.nix
@@ -0,0 +1,124 @@
+# Test for allowUnfreePackages merging across multiple modules
+# Run with: nix-build -A nixosTests.nixpkgs-config-allow-unfree --show-trace
+
+{
+  evalMinimalConfig,
+  lib,
+}:
+let
+  eval =
+    modules:
+    evalMinimalConfig {
+      imports = [
+        ../nixpkgs.nix
+      ]
+      ++ modules;
+    };
+
+  getConfig = evalResult: evalResult.config.nixpkgs.config;
+
+  sortList = list: builtins.sort builtins.lessThan list;
+
+  assertEquals =
+    expected: actual:
+    let
+      prettyExpected = lib.generators.toPretty { } expected;
+      prettyActual = lib.generators.toPretty { } actual;
+    in
+    lib.assertMsg (expected == actual) "Expected ${prettyExpected} but got ${prettyActual}";
+
+  assertUnfreePackages =
+    listOfModules: expectedPackages:
+    let
+      config = getConfig (eval listOfModules);
+      actualAllowedUnfreePackages = sortList config.allowUnfreePackages;
+    in
+    assertEquals expectedPackages actualAllowedUnfreePackages;
+in
+lib.recurseIntoAttrs {
+
+  singleModuleTest =
+    assertUnfreePackages
+      [
+        {
+          nixpkgs.config.allowUnfreePackages = [
+            "package1"
+            "package2"
+          ];
+        }
+      ]
+      [
+        "package1"
+        "package2"
+      ];
+
+  multipleModulesMerging =
+    assertUnfreePackages
+      [
+        {
+          _file = "module1.nix";
+          nixpkgs.config.allowUnfreePackages = [
+            "package1"
+            "package2"
+          ];
+        }
+        {
+          _file = "module2.nix";
+          nixpkgs.config.allowUnfreePackages = [
+            "package3"
+            "package4"
+          ];
+        }
+        {
+          _file = "module3.nix";
+          nixpkgs.config.allowUnfreePackages = [ "package5" ];
+        }
+      ]
+      [
+        "package1"
+        "package2"
+        "package3"
+        "package4"
+        "package5"
+      ];
+
+  overlappingPackagesMerging =
+    assertUnfreePackages
+      [
+        {
+          _file = "moduleA.nix";
+          nixpkgs.config.allowUnfreePackages = [
+            "shared-package"
+            "unique-a"
+          ];
+        }
+        {
+          _file = "moduleB.nix";
+          nixpkgs.config.allowUnfreePackages = [
+            "shared-package"
+            "unique-b"
+          ];
+        }
+      ]
+      [
+        "shared-package"
+        "shared-package"
+        "unique-a"
+        "unique-b"
+      ];
+
+  emptyListMerging =
+    assertUnfreePackages
+      [
+        {
+          _file = "empty.nix";
+          nixpkgs.config.allowUnfreePackages = [ ];
+        }
+        {
+          _file = "non-empty.nix";
+          nixpkgs.config.allowUnfreePackages = [ "some-package" ];
+        }
+      ]
+      [ "some-package" ];
+
+}
diff --git a/nixos/tests/all-tests.nix b/nixos/tests/all-tests.nix
index bbb72047c109..42582b9ed737 100644
--- a/nixos/tests/all-tests.nix
+++ b/nixos/tests/all-tests.nix
@@ -1103,6 +1103,13 @@ in
     imports = [ ./nixos-rebuild-target-host.nix ];
   };
   nixpkgs = pkgs.callPackage ../modules/misc/nixpkgs/test.nix { inherit evalMinimalConfig; };
+  nixpkgs-config-allow-unfree =
+    pkgs.callPackage ../modules/misc/nixpkgs/test-nixpkgs-config-allow-unfree.nix
+      { inherit evalMinimalConfig; };
+  nixpkgs-config-allow-unfree-packages-and-predicate =
+    pkgs.callPackage ../../pkgs/stdenv/generic/check-meta-test.nix
+      { };
+  nixseparatedebuginfod = runTest ./nixseparatedebuginfod.nix;
   nixseparatedebuginfod2 = runTest ./nixseparatedebuginfod2.nix;
   node-red = runTest ./node-red.nix;
   nomad = runTest ./nomad.nix;
diff --git a/pkgs/stdenv/generic/check-meta-test.nix b/pkgs/stdenv/generic/check-meta-test.nix
new file mode 100644
index 000000000000..0dedb25afbd2
--- /dev/null
+++ b/pkgs/stdenv/generic/check-meta-test.nix
@@ -0,0 +1,128 @@
+# Execute with
+#   nix-build -A nixosTests.nixpkgs-config-allow-unfree-packages-and-predicate --show-trace
+#
+# This test exercises the interaction between:
+#
+#   - nixos/modules/misc/nixpkgs.nix  (config merging, esp. allowUnfreePackages)
+#   - pkgs/stdenv/generic/check-meta.nix (allowUnfreePredicate logic)
+#
+# It checks how:
+#
+#   * config.allowUnfreePackages
+#   * config.allowUnfreePredicate
+#
+# interact to determine whether unfree packages are allowed.
+{
+  lib,
+  pkgs,
+}:
+
+let
+  inherit (lib)
+    assertMsg
+    generators
+    licenses
+    recurseIntoAttrs
+    ;
+
+  mkUnfreePkg = name: {
+    pname = name;
+    version = "1.0";
+    meta.license = licenses.unfree;
+  };
+  assertValidity =
+    {
+      nixpkgsConfig,
+      pkg,
+      expected ? true,
+    }:
+    let
+      testPkgs = import ../../.. {
+        system = pkgs.stdenv.hostPlatform.system;
+        config = nixpkgsConfig;
+      };
+      checkMeta = testPkgs.callPackage ./check-meta.nix { };
+      tryEval = expression: builtins.tryEval (builtins.deepSeq expression expression);
+      actual = tryEval (
+        checkMeta.assertValidity {
+          meta = pkg.meta;
+          attrs = pkg;
+        }
+      );
+      toPretty = generators.toPretty { };
+    in
+    assertMsg (actual.success == expected) ''
+      Expected validity of package ${lib.getName pkg} to be ${toPretty expected},
+      but got ${toPretty actual} with config:
+      ${toPretty nixpkgsConfig}
+    '';
+
+  runAssertions = assertions: lib.deepSeq assertions "";
+
+in
+recurseIntoAttrs {
+
+  allowOnlyFreePackagesByDefault = assertValidity {
+    nixpkgsConfig = { };
+    pkg = mkUnfreePkg "forbidden";
+    expected = false;
+  };
+
+  allowAllUnfreePackages = assertValidity {
+    nixpkgsConfig = {
+      allowUnfree = true;
+    };
+    pkg = mkUnfreePkg "allowed";
+  };
+
+  allowUnfreePackagesWithPredicate =
+    let
+      nixpkgsConfig = {
+        allowUnfreePredicate = pkg: lib.getName pkg == "allowed-by-predicate";
+      };
+    in
+    [
+      (assertValidity {
+        inherit nixpkgsConfig;
+        pkg = mkUnfreePkg "allowed-by-predicate";
+      })
+      (assertValidity {
+        inherit nixpkgsConfig;
+        pkg = mkUnfreePkg "allowed-by-nothing";
+        expected = false;
+      })
+    ];
+
+  allowUnfreeWithPackages = runAssertions [
+    (assertValidity {
+      nixpkgsConfig = {
+        allowUnfreePackages = [ "unfree" ];
+      };
+      pkg = mkUnfreePkg "unfree";
+      expected = true;
+    })
+  ];
+
+  allowUnfreePackagesOrPredicate =
+    let
+      nixpkgsConfig = {
+        allowUnfreePackages = [ "allowed-by-packages" ];
+        allowUnfreePredicate = pkg: lib.getName pkg == "allowed-by-predicate";
+      };
+    in
+    runAssertions [
+      (assertValidity {
+        inherit nixpkgsConfig;
+        pkg = mkUnfreePkg "allowed-by-packages";
+      })
+      (assertValidity {
+        inherit nixpkgsConfig;
+        pkg = mkUnfreePkg "allowed-by-predicate";
+      })
+      (assertValidity {
+        inherit nixpkgsConfig;
+        pkg = mkUnfreePkg "forbidden";
+        expected = false;
+      })
+    ];
+}
diff --git a/pkgs/stdenv/generic/check-meta.nix b/pkgs/stdenv/generic/check-meta.nix
index 688429685863..ae868ab14ac1 100644
--- a/pkgs/stdenv/generic/check-meta.nix
+++ b/pkgs/stdenv/generic/check-meta.nix
@@ -160,8 +160,17 @@ let
   # }
   # Defaults to allow all names defined in config.allowUnfreePackages
   allowUnfreePredicate =
-    config.allowUnfreePredicate
-      or (pkg: builtins.elem (lib.getName pkg) config.allowUnfreePackages or [ ]);
+    let
+      listPredicate = pkg: builtins.elem (lib.getName pkg) (config.allowUnfreePackages or [ ]);
+
+      # Be robust against misconfigured allowUnfreePredicate values such as null
+      explicitPredicate =
+        let
+          raw = config.allowUnfreePredicate or null;
+        in
+        if builtins.isFunction raw then raw else (_: false);
+    in
+    pkg: (listPredicate pkg) || (explicitPredicate pkg);
 
   # Check whether unfree packages are allowed and if not, whether the
   # package has an unfree license and is not explicitly allowed by the