diff --git a/structutil/struct.go b/structutil/struct.go
index eb053b7..69e1288 100644
--- a/structutil/struct.go
+++ b/structutil/struct.go
@@ -1,8 +1,9 @@
 package structutil
 
 import (
-	"github.com/duke-git/lancet/v2/pointer"
 	"reflect"
+
+	"github.com/duke-git/lancet/v2/pointer"
 )
 
 // DefaultTagName is the default tag for struct fields to lookup.
@@ -49,7 +50,7 @@ func New(value any) *Struct {
 // Only the exported fields of a struct can be converted.
 func (s *Struct) ToMap() (map[string]any, error) {
 	if !s.IsStruct() {
-		return nil, ErrInvalidStruct(s)
+		return nil, errInvalidStruct(s)
 	}
 
 	result := make(map[string]any)
diff --git a/structutil/error.go b/structutil/struct_internal.go
similarity index 68%
rename from structutil/error.go
rename to structutil/struct_internal.go
index 8b808e1..ada0dd2 100644
--- a/structutil/error.go
+++ b/structutil/struct_internal.go
@@ -2,6 +2,6 @@ package structutil
 
 import "fmt"
 
-func ErrInvalidStruct(v any) error {
+func errInvalidStruct(v any) error {
 	return fmt.Errorf("invalid struct %v", v)
 }