diff --git a/src/BaseSelect/index.tsx b/src/BaseSelect/index.tsx
index d19ee824..f8a17d4f 100644
--- a/src/BaseSelect/index.tsx
+++ b/src/BaseSelect/index.tsx
@@ -129,7 +129,7 @@ export type BaseSelectPropsWithoutPrivate = Omit<BaseSelectProps, keyof BaseSele
 
 export interface BaseSelectProps extends BaseSelectPrivateProps, React.AriaAttributes {
   // Style
-  className?: string;
+  className?: string | undefined;
   style?: React.CSSProperties;
   classNames?: Partial<Record<BaseSelectSemanticName, string>>;
   styles?: Partial<Record<BaseSelectSemanticName, React.CSSProperties>>;
@@ -195,7 +195,7 @@ export interface BaseSelectProps extends BaseSelectPrivateProps, React.AriaAttri
   transitionName?: string;
 
   popupStyle?: React.CSSProperties;
-  popupClassName?: string;
+  popupClassName?: string | undefined;
   popupMatchSelectWidth?: boolean | number;
   popupRender?: (menu: React.ReactElement) => React.ReactElement;
   popupAlign?: AlignType;
diff --git a/src/Select.tsx b/src/Select.tsx
index 39da9041..017d7110 100644
--- a/src/Select.tsx
+++ b/src/Select.tsx
@@ -90,7 +90,7 @@ export interface FieldNames {
 
 export interface BaseOptionType {
   disabled?: boolean;
-  className?: string;
+  className?: string | undefined;
   title?: string;
   [name: string]: any;
 }