diff --git a/index.d.ts b/index.d.ts
index 2989496..895fc9e 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -8,12 +8,12 @@ export type LocaleData = {
 };
 
 export function registerLocale(localeData: LocaleData): void;
-export function alpha2ToAlpha3(alpha2: string): string;
-export function alpha2ToNumeric(alpha2: string): number;
-export function alpha3ToAlpha2(alpha3: string): string;
-export function alpha3ToNumeric(alpha3: string): number;
-export function numericToAlpha2(numeric: number | string): string;
-export function numericToAlpha3(numeric: number | string): string;
+export function alpha2ToAlpha3(alpha2: string): string | undefined;
+export function alpha2ToNumeric(alpha2: string): number | undefined;
+export function alpha3ToAlpha2(alpha3: string): string | undefined;
+export function alpha3ToNumeric(alpha3: string): number | undefined;
+export function numericToAlpha2(numeric: number | string): string | undefined;
+export function numericToAlpha3(numeric: number | string): string | undefined;
 /**
  * Returns object map where key is alpha 2 code and value is alpha 3 code
  */
@@ -26,13 +26,13 @@ export function getAlpha3Codes(): { [alpha3Key: string]: string };
  * Returns object map where key is numeric code and value is alpha 2 code
  */
 export function getNumericCodes(): { [numericKey: number]: string };
-export function getName(alpha2orAlpha3orNumeric: string | number, lang: string): string;
+export function getName(alpha2orAlpha3orNumeric: string | number, lang: string): string | undefined;
 export function getNames(lang: string): LocalizedNationalityNames;
-export function toAlpha3(alpha2orNumeric: number | string): string;
-export function toAlpha2(alpha3orNumeric: number | string): string;
-export function getAlpha2Code(name: string, lang: string): string;
-export function getSimpleAlpha2Code(name: string, lang: string): string;
-export function getAlpha3Code(name: string, lang: string): string;
-export function getSimpleAlpha3Code(name: string, lang: string): string;
+export function toAlpha3(alpha2orNumeric: number | string): string | undefined;
+export function toAlpha2(alpha3orNumeric: number | string): string | undefined;
+export function getAlpha2Code(name: string, lang: string): string | undefined;
+export function getSimpleAlpha2Code(name: string, lang: string): string | undefined;
+export function getAlpha3Code(name: string, lang: string): string | undefined;
+export function getSimpleAlpha3Code(name: string, lang: string): string | undefined;
 export function langs(): string[];
 export function isValid(alpha2orAlpha3orNumeric: string | number): boolean;
\ No newline at end of file