diff --git a/lib/bandit/websocket/frame.ex b/lib/bandit/websocket/frame.ex
index a4938003..3c047bb6 100644
--- a/lib/bandit/websocket/frame.ex
+++ b/lib/bandit/websocket/frame.ex
@@ -96,17 +96,19 @@ defmodule Bandit.WebSocket.Frame do
   defp mask_and_length(length), do: <<0::1, 127::7, length::64>>
 
   # Note that masking is an involution, so we don't need a separate unmask function
-  def mask(payload, mask, acc \\ <<>>)
-
-  def mask(payload, mask, acc) when is_integer(mask), do: mask(payload, <<mask::32>>, acc)
+  def mask(payload, mask) do
+    payload
+    |> do_mask(<<mask::32>>, [])
+    |> IO.iodata_to_binary()
+  end
 
-  def mask(<<h::32, rest::binary>>, <<mask::32>>, acc) do
-    mask(rest, mask, acc <> <<Bitwise.bxor(h, mask)::32>>)
+  defp do_mask(<<h::32, rest::binary>>, <<int_mask::32>> = mask, acc) do
+    do_mask(rest, mask, [acc, <<Bitwise.bxor(h, int_mask)::32>>])
   end
 
-  def mask(<<h::8, rest::binary>>, <<current::8, mask::24>>, acc) do
-    mask(rest, <<mask::24, current::8>>, acc <> <<Bitwise.bxor(h, current)::8>>)
+  defp do_mask(<<h::8, rest::binary>>, <<current::8, mask::24>>, acc) do
+    do_mask(rest, <<mask::24, current::8>>, [acc, <<Bitwise.bxor(h, current)::8>>])
   end
 
-  def mask(<<>>, _mask, acc), do: acc
+  defp do_mask(<<>>, _mask, acc), do: acc
 end