diff --git a/lib/open_api_spex/cast/utils.ex b/lib/open_api_spex/cast/utils.ex
index 6c3e3923..084a9ca9 100644
--- a/lib/open_api_spex/cast/utils.ex
+++ b/lib/open_api_spex/cast/utils.ex
@@ -5,8 +5,11 @@ defmodule OpenApiSpex.Cast.Utils do
 
   # Merge 2 maps considering as equal keys that are atom or string representation
   # of that atom. Atom keys takes precedence over string ones.
+  # Struct arguments are demoted to plain maps first: composite casts
+  # (`allOf`/`anyOf`) can receive struct results from `oneOf`/discriminator
+  # members, and reducing over a struct raises `Protocol.UndefinedError`.
   def merge_maps(map1, map2) do
-    result = Map.merge(map1, map2)
+    result = Map.merge(plain_map(map1), plain_map(map2))
 
     Enum.reduce(result, result, fn
       {k, _v}, result when is_atom(k) -> Map.delete(result, to_string(k))
@@ -14,6 +17,9 @@ defmodule OpenApiSpex.Cast.Utils do
     end)
   end
 
+  defp plain_map(%_{} = struct), do: Map.from_struct(struct)
+  defp plain_map(map), do: map
+
   def check_required_fields(%{value: input_map} = ctx), do: check_required_fields(ctx, input_map)
 
   def check_required_fields(ctx, %{} = input_map) do
diff --git a/test/cast/all_of_test.exs b/test/cast/all_of_test.exs
index 9f2fbabe..ce6cb7aa 100644
--- a/test/cast/all_of_test.exs
+++ b/test/cast/all_of_test.exs
@@ -34,6 +34,14 @@ defmodule OpenApiSpex.CastAllOfTest do
                "Failed to cast value as Age. Value must be castable using `allOf` schemas listed."
     end
 
+    test "allOf wrapper around a oneOf member whose branch casts to a struct" do
+      union = %Schema{oneOf: [OpenApiSpexTest.Schemas.User.schema()]}
+      schema = %Schema{allOf: [union]}
+      value = %{"name" => "Joe", "email" => "joe@example.com", "password" => "12345678"}
+
+      assert {:ok, %{name: "Joe"}} = cast(value: value, schema: schema)
+    end
+
     test "a more sophisticated example" do
       dog = %{"bark" => "woof", "pet_type" => "Dog"}
       TestAssertions.assert_schema(dog, "Dog", OpenApiSpexTest.ApiSpec.spec())