diff --git a/lib/ash/changeset/changeset.ex b/lib/ash/changeset/changeset.ex
index 27d37d9f2..6ffab8dc6 100644
--- a/lib/ash/changeset/changeset.ex
+++ b/lib/ash/changeset/changeset.ex
@@ -6612,29 +6612,33 @@ defmodule Ash.Changeset do
 
   defp do_set_argument(changeset, argument, value, store_casted? \\ false) do
     if changeset.action do
-      argument =
+      action_argument =
         Enum.find(
           changeset.action.arguments,
           &(&1.name == argument || to_string(&1.name) == argument)
         )
 
-      if argument do
-        with value <- Ash.Type.Helpers.handle_indexed_maps(argument.type, value),
+      if action_argument do
+        with value <- Ash.Type.Helpers.handle_indexed_maps(action_argument.type, value),
              constraints <-
-               Ash.Type.include_source(argument.type, changeset, argument.constraints),
+               Ash.Type.include_source(
+                 action_argument.type,
+                 changeset,
+                 action_argument.constraints
+               ),
              {:ok, casted} <-
-               Ash.Type.cast_input(argument.type, value, constraints),
+               Ash.Type.cast_input(action_argument.type, value, constraints),
              {{:ok, casted}, _last_val} <-
-               {Ash.Type.apply_constraints(argument.type, casted, constraints), casted} do
-          %{changeset | arguments: Map.put(changeset.arguments, argument.name, casted)}
-          |> store_casted_argument(argument.name, casted, store_casted?)
+               {Ash.Type.apply_constraints(action_argument.type, casted, constraints), casted} do
+          %{changeset | arguments: Map.put(changeset.arguments, action_argument.name, casted)}
+          |> store_casted_argument(action_argument.name, casted, store_casted?)
         else
           {:error, error} ->
-            add_invalid_errors(value, :argument, changeset, argument, error)
+            add_invalid_errors(value, :argument, changeset, action_argument, error)
 
           {{:error, error}, last_val} ->
-            add_invalid_errors(value, :argument, changeset, argument, error)
-            |> store_casted_argument(argument.name, last_val, store_casted?)
+            add_invalid_errors(value, :argument, changeset, action_argument, error)
+            |> store_casted_argument(action_argument.name, last_val, store_casted?)
         end
       else
         %{changeset | arguments: Map.put(changeset.arguments, argument, value)}
diff --git a/test/changeset/changeset_test.exs b/test/changeset/changeset_test.exs
index daec03d27..706767b7f 100644
--- a/test/changeset/changeset_test.exs
+++ b/test/changeset/changeset_test.exs
@@ -1380,6 +1380,16 @@ defmodule Ash.Test.Changeset.ChangesetTest do
       assert Ash.Changeset.get_argument(changeset, :true_optional_argument) == true
       assert Ash.Changeset.get_argument(changeset, :false_optional_argument) == false
     end
+
+    test "arguments not defined on the action are stored under their name" do
+      changeset =
+        Category
+        |> Ash.Changeset.for_create(:create_with_confirmation)
+        |> Ash.Changeset.force_set_argument(:unknown_argument, "foo")
+
+      assert changeset.arguments[:unknown_argument] == "foo"
+      assert is_nil(changeset.arguments[nil])
+    end
   end
 
   describe "validation keyword errors" do