diff --git a/encoding_test.go b/encoding_test.go
index 61199c0..3e67a30 100644
--- a/encoding_test.go
+++ b/encoding_test.go
@@ -36,4 +36,70 @@ func TestDefaultCodecRegistry(t *testing.T) {
 
 		assert.Equal(t, c, decoder)
 	})
+
+	t.Run("CodecNotFound", func(t *testing.T) {
+		registry := NewCodecRegistry()
+
+		_, err := registry.Encoder("myformat")
+		require.Error(t, err)
+
+		_, err = registry.Decoder("myformat")
+		require.Error(t, err)
+	})
+
+	t.Run("FormatIsCaseInsensitive", func(t *testing.T) {
+		registry := NewCodecRegistry()
+
+		c := codec{}
+
+		err := registry.RegisterCodec("MYFORMAT", c)
+		require.NoError(t, err)
+
+		{
+			encoder, err := registry.Encoder("myformat")
+			require.NoError(t, err)
+
+			assert.Equal(t, c, encoder)
+		}
+
+		{
+			encoder, err := registry.Encoder("MYFORMAT")
+			require.NoError(t, err)
+
+			assert.Equal(t, c, encoder)
+		}
+
+		{
+			decoder, err := registry.Decoder("myformat")
+			require.NoError(t, err)
+
+			assert.Equal(t, c, decoder)
+		}
+
+		{
+			decoder, err := registry.Decoder("MYFORMAT")
+			require.NoError(t, err)
+
+			assert.Equal(t, c, decoder)
+		}
+	})
+
+	t.Run("OverrideDefault", func(t *testing.T) {
+		registry := NewCodecRegistry()
+
+		c := codec{}
+
+		err := registry.RegisterCodec("yaml", c)
+		require.NoError(t, err)
+
+		encoder, err := registry.Encoder("yaml")
+		require.NoError(t, err)
+
+		assert.Equal(t, c, encoder)
+
+		decoder, err := registry.Decoder("yaml")
+		require.NoError(t, err)
+
+		assert.Equal(t, c, decoder)
+	})
 }