diff --git a/lib/ecto/changeset.ex b/lib/ecto/changeset.ex
index f79066955f..09dd81ebb4 100644
--- a/lib/ecto/changeset.ex
+++ b/lib/ecto/changeset.ex
@@ -4323,11 +4323,13 @@ defmodule Ecto.Changeset do
 
     acc = Map.put(acc, key, relation_changed)
 
-    with %Ecto.Association.BelongsTo{related_key: related_key} <- relation,
-         %{^related_key => id} <- relation_changed do
-      Map.put(acc, relation.owner_key, id)
-    else
-      _ -> acc
+    case relation do
+      %Ecto.Association.BelongsTo{related_key: related_key, owner_key: owner_key} ->
+        id = relation_changed && Map.fetch!(relation_changed, related_key)
+        Map.put(acc, owner_key, id)
+
+      _ ->
+        acc
     end
   end
 
diff --git a/test/ecto/changeset_test.exs b/test/ecto/changeset_test.exs
index 3f1b7910b6..d4b2054ead 100644
--- a/test/ecto/changeset_test.exs
+++ b/test/ecto/changeset_test.exs
@@ -1083,8 +1083,10 @@ defmodule Ecto.ChangesetTest do
     assert changed_post.title == "foo"
     assert changed_post.category_id == category.category_id
 
+    post_with_category = %Post{category_id: 5}
+
     changeset =
-      post
+      post_with_category
       |> changeset(%{"title" => "foo"})
       |> put_assoc(:category, nil)