diff --git a/entry.go b/entry.go
index 3158adc..0cb2690 100644
--- a/entry.go
+++ b/entry.go
@@ -39,6 +39,10 @@ func (e *Entry) checkAuthors() error {
 	return nil
 }
 
+func (e *Entry) AddExtension(name string, value any) {
+	e.Extensions = append(e.Extensions, &ExtensionElement{XMLName: xml.Name{Local: name}, Value: value})
+}
+
 func (e *Entry) Check() error {
 	if e.ID == nil {
 		return errors.New("no id element of entry")
@@ -132,7 +136,3 @@ func (e *Entry) Check() error {
 
 	return nil
 }
-
-func (e *Entry) AddExtension(name string, value any) {
-	e.Extensions = append(e.Extensions, &ExtensionElement{XMLName: xml.Name{Local: name}, Value: value})
-}
diff --git a/feed.go b/feed.go
index c492c2c..3a3379e 100644
--- a/feed.go
+++ b/feed.go
@@ -25,6 +25,10 @@ type Feed struct {
 	Entries      []*Entry            `xml:"entry,omitempty"`
 }
 
+func (f *Feed) AddExtension(name string, value any) {
+	f.Extensions = append(f.Extensions, &ExtensionElement{XMLName: xml.Name{Local: name}, Value: value})
+}
+
 func (f *Feed) Check() error {
 	if f.ID == nil {
 		return errors.New("no id element of feed")
@@ -156,10 +160,6 @@ func (f *Feed) Standardize() {
 	}
 }
 
-func (f *Feed) AddExtension(name string, value any) {
-	f.Extensions = append(f.Extensions, &ExtensionElement{XMLName: xml.Name{Local: name}, Value: value})
-}
-
 func (f *Feed) ToXML(encoding string) (string, error) {
 	xml, err := xml.MarshalIndent(f, "", "  ")
 	if err != nil {