diff --git a/lib/modules.nix b/lib/modules.nix
index 8b9a925d718a..d7125e60b8e5 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -1185,29 +1185,32 @@ let
         throw
           "The option `${showOption loc}' was accessed but has no value defined. Try setting the option.";
 
-    ensureMergedValueEnvelope =
-      v:
-      if attrNames v == attrNames defaultCheckedAndMerged then
-        v
-      else
-        throw "Invalid 'type.merge.v2' of type: '${type.name}' must return exactly the following attributes: ${builtins.toJSON (attrNames defaultCheckedAndMerged)} but got ${builtins.toJSON (attrNames v)}";
-
-    defaultCheckedAndMerged = {
-      headError = null;
-      value = mergedValue;
-      valueMeta = { };
-    };
-
-    checkedAndMerged = ensureMergedValueEnvelope (
-      if type.merge ? v2 then
-        type.merge.v2 {
-          inherit loc;
-          defs = defsFinal;
-        }
-      else
-        defaultCheckedAndMerged
-
-    );
+    checkedAndMerged =
+      (
+        # This function (which is immediately applied) checks that type.merge
+        # returns the proper attrset.
+        # Once use of the merge.v2 feature has propagated, consider removing this
+        # for an estimated one thousandth performance improvement (NixOS by nr.thunks).
+        {
+          headError,
+          value,
+          valueMeta,
+        }@args:
+        args
+      )
+        (
+          if type.merge ? v2 then
+            type.merge.v2 {
+              inherit loc;
+              defs = defsFinal;
+            }
+          else
+            {
+              headError = null;
+              value = mergedValue;
+              valueMeta = { };
+            }
+        );
 
     isDefined = defsFinal != [ ];
 
diff --git a/lib/types.nix b/lib/types.nix
index a6b3e6fb83fb..40d0eb6c49f8 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -1503,23 +1503,27 @@ let
             v2 =
               { loc, defs }:
               let
-                isMergeV2 = coercedType.merge ? v2;
-                coerceDef =
-                  def:
-                  let
-                    merged = coercedType.merge.v2 {
-                      inherit loc;
-                      defs = [ def ];
-                    };
-                  in
-                  if isMergeV2 then
-                    if merged.headError == null then coerceFunc def.value else def.value
-                  else if coercedType.check def.value then
-                    coerceFunc def.value
-                  else
-                    def.value;
-
-                finalDefs = (map (def: def // { value = coerceDef def; }) defs);
+                finalDefs = (
+                  map (
+                    def:
+                    def
+                    // {
+                      value =
+                        let
+                          merged = coercedType.merge.v2 {
+                            inherit loc;
+                            defs = [ def ];
+                          };
+                        in
+                        if coercedType.merge ? v2 then
+                          if merged.headError == null then coerceFunc def.value else def.value
+                        else if coercedType.check def.value then
+                          coerceFunc def.value
+                        else
+                          def.value;
+                    }
+                  ) defs
+                );
               in
               if finalType.merge ? v2 then
                 finalType.merge.v2 {
@@ -1530,7 +1534,7 @@ let
                 {
                   value = finalType.merge loc finalDefs;
                   valueMeta = { };
-                  headError = checkDefsForError check loc finalDefs;
+                  headError = checkDefsForError check loc defs;
                 };
           };
           emptyValue = finalType.emptyValue;