lib/pleroma/web/api_spec/operations/twitter_util_operation.ex

@@ -293,7 +293,8 @@ def add_alias_operation do
             }
           }),
         400 => Operation.response("Error", "application/json", ApiError),
-        403 => Operation.response("Error", "application/json", ApiError)
+        403 => Operation.response("Error", "application/json", ApiError),
+        404 => Operation.response("Error", "application/json", ApiError)
       }
     }
   end

lib/pleroma/web/twitter_api/controllers/util_controller.ex

@@ -196,6 +196,11 @@ def add_alias(%{assigns: %{user: user}, body_params: body_params} = conn, _) do
          {:ok, _user} <- user |> User.add_alias(alias_user) do
       json(conn, %{status: "success"})
     else
+      {:not_found, _} ->
+        conn
+        |> put_status(404)
+        |> json(%{error: "Target account does not exist."})
+
       {:error, error} ->
         json(conn, %{error: error})
     end