diff --git a/structs/struct.go b/structs/struct.go
index 8c080d2..14207ed 100644
--- a/structs/struct.go
+++ b/structs/struct.go
@@ -2,6 +2,7 @@
 package structs
 
 import (
+	"fmt"
 	"reflect"
 
 	"github.com/duke-git/lancet/v2/pointer"
@@ -61,7 +62,7 @@ func New(value any, tagName ...string) *Struct {
 // ToMap convert the exported fields of a struct to map.
 func (s *Struct) ToMap() (map[string]any, error) {
 	if !s.IsStruct() {
-		return nil, errInvalidStruct(s)
+		return nil, fmt.Errorf("invalid struct %v", s)
 	}
 
 	result := make(map[string]any)
diff --git a/structs/struct_internal.go b/structs/struct_internal.go
deleted file mode 100644
index bc147cc..0000000
--- a/structs/struct_internal.go
+++ /dev/null
@@ -1,7 +0,0 @@
-package structs
-
-import "fmt"
-
-func errInvalidStruct(v any) error {
-	return fmt.Errorf("invalid struct %v", v)
-}