diff --git a/lib/harald/hci.ex b/lib/harald/hci.ex
index 6a1b73f..5c21777 100644
--- a/lib/harald/hci.ex
+++ b/lib/harald/hci.ex
@@ -91,7 +91,11 @@ defmodule Harald.HCI do
 
   @impl Serializable
   def deserialize(<<4, rest::binary>>) do
-    Event.deserialize(rest)
+    case Event.deserialize(rest) do
+      {:ok, _} = ret -> ret
+      {:error, bin} when is_binary(bin) -> {:error, <<4, bin::binary>>}
+      {:error, data} -> {:error, data}
+    end
   end
 
   def deserialize(bin), do: {:error, bin}
diff --git a/lib/harald/serializable.ex b/lib/harald/serializable.ex
index c7e3f3a..ad4bc39 100644
--- a/lib/harald/serializable.ex
+++ b/lib/harald/serializable.ex
@@ -6,4 +6,21 @@ defmodule Harald.Serializable do
   @callback serialize(term()) :: {:ok, binary()} | {:error, term()}
 
   @callback deserialize(binary()) :: {:ok, term()} | {:error, term()}
+
+  @doc """
+  Asserts that `bin` will serialize symmetrically.
+
+  If `bin` deserializes into `{:ok, data}`, `data` should serialize perfectly back into `bin`.
+  """
+  defmacro assert_symmetry(mod, bin) do
+    quote bind_quoted: [bin: bin, mod: mod] do
+      import ExUnit.Assertions, only: [assert: 1, assert: 2]
+
+      case mod.deserialize(bin) do
+        {:ok, data} -> assert {:ok, bin} == mod.serialize(data)
+        {:error, data} when not is_binary(data) -> true
+        {:error, bin2} -> assert bin == bin2
+      end
+    end
+  end
 end
diff --git a/test/harald/hci_test.exs b/test/harald/hci_test.exs
index ebd9fd1..0d10f9e 100644
--- a/test/harald/hci_test.exs
+++ b/test/harald/hci_test.exs
@@ -3,6 +3,7 @@ defmodule Harald.HCITest do
   use ExUnitProperties
   alias Harald.Generators.HCI, as: HCIGen
   alias Harald.HCI
+  require Harald.Serializable, as: Serializable
 
   doctest Harald.HCI, import: true
 
@@ -61,11 +62,8 @@ defmodule Harald.HCITest do
       end
     end
 
-    check all parameters <- StreamData.binary() do
-      case HCI.deserialize(parameters) do
-        {:ok, data} -> assert {:ok, parameters} == HCI.serialize(data)
-        {:error, _} -> :ok
-      end
+    check all bin <- StreamData.binary() do
+      Serializable.assert_symmetry(HCI, bin)
     end
   end
 end