diff --git a/migrator.go b/migrator.go
index e4d8e92..c9a5761 100644
--- a/migrator.go
+++ b/migrator.go
@@ -326,8 +326,7 @@ func (m Migrator) AlterColumn(value interface{}, field string) error {
 							return err
 						}
 					} else {
-						if err := m.DB.Exec("ALTER TABLE ? ALTER COLUMN ? TYPE ?"+m.genUsingExpression(fileType.SQL, fieldColumnType.DatabaseTypeName()),
-							m.CurrentTable(stmt), clause.Column{Name: field.DBName}, fileType, clause.Column{Name: field.DBName}, fileType).Error; err != nil {
+						if err := m.modifyColumn(stmt, field, fileType, fieldColumnType); err != nil {
 							return err
 						}
 					}
@@ -387,14 +386,27 @@ func (m Migrator) AlterColumn(value interface{}, field string) error {
 	return nil
 }
 
-func (m Migrator) genUsingExpression(targetType, sourceType string) string {
-	if targetType == "boolean" {
-		switch sourceType {
+func (m Migrator) modifyColumn(stmt *gorm.Statement, field *schema.Field, targetType clause.Expr, existingColumn *migrator.ColumnType) error {
+	alterSQL := "ALTER TABLE ? ALTER COLUMN ? TYPE ? USING ?::?"
+	isUncastableDefaultValue := false
+
+	if targetType.SQL == "boolean" {
+		switch existingColumn.DatabaseTypeName() {
 		case "int2", "int8", "numeric":
-			return " USING ?::INT::?"
+			alterSQL = "ALTER TABLE ? ALTER COLUMN ? TYPE ? USING ?::int::?"
+		}
+		isUncastableDefaultValue = true
+	}
+
+	if dv, _ := existingColumn.DefaultValue(); dv != "" && isUncastableDefaultValue {
+		if err := m.DB.Exec("ALTER TABLE ? ALTER COLUMN ? DROP DEFAULT", m.CurrentTable(stmt), clause.Column{Name: field.DBName}).Error; err != nil {
+			return err
 		}
 	}
-	return " USING ?::?"
+	if err := m.DB.Exec(alterSQL, m.CurrentTable(stmt), clause.Column{Name: field.DBName}, targetType, clause.Column{Name: field.DBName}, targetType).Error; err != nil {
+		return err
+	}
+	return nil
 }
 
 func (m Migrator) HasConstraint(value interface{}, name string) bool {