diff --git a/README.md b/README.md
index b01ea64..37cbb8a 100644
--- a/README.md
+++ b/README.md
@@ -18,7 +18,7 @@ import "sourcecode.social/reiver/go-nul"
 
 //
 
-var op nul.Optional[string] // the default value is nothing.
+var op nul.Nullable[string] // the default value is nothing.
 
 // ...
 
diff --git a/nullable_filter_test.go b/nullable_filter_test.go
index 8af3712..c85af62 100644
--- a/nullable_filter_test.go
+++ b/nullable_filter_test.go
@@ -6,7 +6,7 @@ import (
 	"sourcecode.social/reiver/go-nul"
 )
 
-func TestOptional_Filter_int(t *testing.T) {
+func TestNullable_Filter_int(t *testing.T) {
 
 	tests := []struct{
 		Nullable nul.Nullable[int]
diff --git a/nullable_get_test.go b/nullable_get_test.go
index d555a5d..b036145 100644
--- a/nullable_get_test.go
+++ b/nullable_get_test.go
@@ -6,7 +6,7 @@ import (
 	"sourcecode.social/reiver/go-nul"
 )
 
-func TestOptional_Get_string(t *testing.T) {
+func TestNullable_Get_string(t *testing.T) {
 
 	tests := []struct{
 		Nullable nul.Nullable[string]
@@ -77,7 +77,7 @@ func TestOptional_Get_string(t *testing.T) {
 	}
 }
 
-func TestOptional_Get_int8(t *testing.T) {
+func TestNullable_Get_int8(t *testing.T) {
 
 	tests := []struct{
 		Nullable nul.Nullable[int8]
@@ -303,7 +303,7 @@ func TestOptional_Get_int8(t *testing.T) {
 	}
 }
 
-func TestOptional_Get_uint8(t *testing.T) {
+func TestNullable_Get_uint8(t *testing.T) {
 
 	tests := []struct{
 		Nullable nul.Nullable[uint8]