diff --git a/lib/bson/types.ex b/lib/bson/types.ex
index 4062a20..7095469 100644
--- a/lib/bson/types.ex
+++ b/lib/bson/types.ex
@@ -124,7 +124,7 @@ defmodule BSON.ObjectId do
     defp e(unquote(int)), do: unquote(char)
   end
 
-  for {char, int} <- Enum.with_index(?A..?F) do
+  for {char, int} <- Enum.with_index(?A..?F, 10) do
     defp d(unquote(char)), do: unquote(int)
   end
 
diff --git a/test/bson/types_test.exs b/test/bson/types_test.exs
index 0d33bbd..f60bc73 100644
--- a/test/bson/types_test.exs
+++ b/test/bson/types_test.exs
@@ -11,6 +11,7 @@ defmodule BSON.TypesTest do
 
   @objectid %BSON.ObjectId{value: <<29, 32, 69, 244, 101, 119, 228, 28, 61, 24, 21, 215>>}
   @string "1d2045f46577e41c3d1815d7"
+  @string_uppercase "1D2045F46577E41C3D1815D7"
   @timestamp DateTime.from_unix!(488_654_324)
 
   test "inspect BSON.ObjectId" do
@@ -39,6 +40,14 @@ defmodule BSON.TypesTest do
     end
   end
 
+  test "BSON.ObjectId.decode!/1 for uppercase HEX" do
+    assert BSON.ObjectId.decode!(@string_uppercase) == @objectid
+
+    assert_raise FunctionClauseError, fn ->
+      BSON.ObjectId.decode!("")
+    end
+  end
+
   test "BSON.ObjectId.encode/1" do
     assert BSON.ObjectId.encode(@objectid) == {:ok, @string}
     assert BSON.ObjectId.encode("") == :error