diff --git a/lib/ecto/adapters/sqlite3/codec.ex b/lib/ecto/adapters/sqlite3/codec.ex
index 11f5c4e..64fb68b 100644
--- a/lib/ecto/adapters/sqlite3/codec.ex
+++ b/lib/ecto/adapters/sqlite3/codec.ex
@@ -98,12 +98,12 @@ defmodule Ecto.Adapters.SQLite3.Codec do
   def bool_encode(false), do: {:ok, 0}
   def bool_encode(true), do: {:ok, 1}
 
+  def decimal_encode(nil), do: {:ok, nil}
+
   def decimal_encode(%Decimal{} = x) do
     {:ok, Decimal.to_string(x, :normal)}
   end
 
-  # def decimal_encode(x), do: {:ok, x}
-
   def time_encode(value) do
     {:ok, value}
   end
diff --git a/test/ecto/adapters/sqlite3/codec_test.exs b/test/ecto/adapters/sqlite3/codec_test.exs
index 2be15cb..ab8532d 100644
--- a/test/ecto/adapters/sqlite3/codec_test.exs
+++ b/test/ecto/adapters/sqlite3/codec_test.exs
@@ -81,6 +81,17 @@ defmodule Ecto.Adapters.SQLite3.CodecTest do
     end
   end
 
+  describe ".decimal_encode/1" do
+    test "nil" do
+      {:ok, nil} = Codec.decimal_encode(nil)
+    end
+
+    test "decimal" do
+      decimal = Decimal.new("2.5")
+      {:ok, "2.5"} = Codec.decimal_encode(decimal)
+    end
+  end
+
   describe ".time_decode/1" do
     test "nil" do
       {:ok, nil} = Codec.time_decode(nil)