diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index c7b661e2e2fe..4eb86ddd4962 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -902,13 +902,16 @@ checkConfigError 'Did you mean .services\.nginx\.virtualHosts\."example\.com"\.s
 # types.attrListOf
 checkConfigOutput '"ok"' config.assertions ./declare-attrList.nix
 checkConfigError 'A definition for option .attrListInt.badValue.a. is not of type .signed integer.. Definition values:' config.attrListIntStrict.badValue ./declare-attrList.nix
-checkConfigError 'A definition for option .attrList.badListElem. is not of type .attribute list of string.. Each list element must be a single-key attribute set.' config.attrListStrict.badListElem ./declare-attrList.nix
+checkConfigError 'A definition for option .attrList.badListElem. is not of type .attribute list of string.. Each list element must be a single-key attribute set, but got 2 keys' config.attrListStrict.badListElem ./declare-attrList.nix
 checkConfigError 'A definition for option .attrList.badString. is not of type .attribute list of string.. TypeError: Definition values:' config.attrListStrict.badString ./declare-attrList.nix
-checkConfigError 'A definition for option .attrList.badListString. is not of type .attribute list of string.. Each list element must be a single-key attribute set.' config.attrListStrict.badListString ./declare-attrList.nix
+checkConfigError 'A definition for option .attrList.badListString. is not of type .attribute list of string.. Each list element must be an attribute set, but got string' config.attrListStrict.badListString ./declare-attrList.nix
 
 # attrListWith valueMeta.definitions: file propagation
 checkConfigError 'the-defs-file\.nix' config.argv ./attrList-valueMeta-definitions-file-diagnostic-forwarding.nix
 
+# attrListOf does not support type merging
+checkConfigError 'The option .merged. in .*/declare-attrList-type-merge.nix. is already declared in .*/declare-attrList-type-merge.nix' config.merged ./declare-attrList-type-merge.nix
+
 cat <<EOF
 ====== module tests ======
 $pass Pass
diff --git a/lib/tests/modules/declare-attrList-type-merge.nix b/lib/tests/modules/declare-attrList-type-merge.nix
new file mode 100644
index 000000000000..482a90e90954
--- /dev/null
+++ b/lib/tests/modules/declare-attrList-type-merge.nix
@@ -0,0 +1,12 @@
+# Test that attrListOf does not support type merging:
+# two declarations of the same option should fail.
+{ lib, ... }:
+let
+  inherit (lib) mkOption types;
+in
+{
+  imports = [
+    { options.merged = mkOption { type = types.attrListOf types.str; }; }
+    { options.merged = mkOption { type = types.attrListOf types.str; }; }
+  ];
+}
diff --git a/lib/types.nix b/lib/types.nix
index f569c3e4f946..3cb3c64baf05 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -876,7 +876,7 @@ rec {
             extractItem =
               file: raw:
               let
-                hasOrder = raw._type or null == "order";
+                hasOrder = isType "order" raw;
                 item = if hasOrder then raw.content else raw;
                 key = head (attrNames item);
                 peeled = peelProperties item.${key};
@@ -888,7 +888,19 @@ rec {
                   prio = if hasOrder then raw.priority else peeled.prio;
                 }
               else
-                throw "A definition for option `${showOption loc}' is not of type `${description}'. Each list element must be a single-key attribute set.";
+                throw "A definition for option `${showOption loc}' is not of type `${description}'. ${
+                  if !isAttrs item then
+                    "Each list element must be an attribute set, but got ${builtins.typeOf item}"
+                  else
+                    "Each list element must be a single-key attribute set, but got ${toString (length (attrNames item))} keys"
+                }.${
+                  showDefs [
+                    {
+                      inherit file;
+                      value = raw;
+                    }
+                  ]
+                }";
 
             # Convert a definition to a flat list of { file, key, value, prio, overridePrio }
             defToItems =
@@ -969,14 +981,7 @@ rec {
           inherit asAttrs mergeAttrValues;
           elemType = elemType.substSubModules m;
         };
-      functor = elemTypeFunctor name { inherit elemType; } // {
-        type =
-          payload:
-          types.attrListWith {
-            inherit asAttrs mergeAttrValues;
-            inherit (payload) elemType;
-          };
-      };
+      typeMerge = t: null; # Disable type merging
       nestedTypes.elemType = elemType;
     };