diff --git a/migrator.go b/migrator.go
index 2a919ca..e658db3 100644
--- a/migrator.go
+++ b/migrator.go
@@ -319,7 +319,7 @@ func (m Migrator) AlterColumn(value interface{}, field string) error {
 				}
 			}
 
-			if uniq, _ := fieldColumnType.Unique(); uniq != field.Unique {
+			if uniq, _ := fieldColumnType.Unique(); !uniq && field.Unique {
 				idxName := clause.Column{Name: m.DB.Config.NamingStrategy.IndexName(stmt.Table, field.DBName)}
 				if err := m.DB.Exec("ALTER TABLE ? ADD CONSTRAINT ? UNIQUE(?)", m.CurrentTable(stmt), idxName, clause.Column{Name: field.DBName}).Error; err != nil {
 					return err