diff --git a/src/index.d.ts b/src/index.d.ts
index 7e02074..93e7d6e 100644
--- a/src/index.d.ts
+++ b/src/index.d.ts
@@ -1,4 +1,4 @@
-export interface FluxStandardAction<Payload, Meta> {
+export interface FluxStandardAction<Payload, Meta = undefined> {
   /**
    * The `type` of an action identifies to the consumer the nature of the action that has occurred.
    * Two actions with the same `type` MUST be strictly equivalent (using `===`)
@@ -26,26 +26,26 @@ export interface FluxStandardAction<Payload, Meta> {
   meta: Meta;
 }
 
-export interface ErrorFluxStandardAction<CustomError extends Error, Meta> extends FluxStandardAction<CustomError, Meta> {
+export interface ErrorFluxStandardAction<CustomError extends Error, Meta = undefined> extends FluxStandardAction<CustomError, Meta> {
   error: true;
 }
 
 /**
  * Alias for FluxStandardAction.
  */
-export type FSA<Payload, Meta> = FluxStandardAction<Payload, Meta>;
+export type FSA<Payload, Meta = undefined> = FluxStandardAction<Payload, Meta>;
 
 /**
  * Alias for ErrorFluxStandardAction.
  */
-export type ErrorFSA<CustomError extends Error, Meta> = ErrorFluxStandardAction<CustomError, Meta>;
+export type ErrorFSA<CustomError extends Error, Meta = undefined> = ErrorFluxStandardAction<CustomError, Meta>;
 
 /**
  * Returns `true` if `action` is FSA compliant.
  */
-export function isFSA<Payload, Meta>(action: any): action is FluxStandardAction<Payload, Meta>;
+export function isFSA<Payload, Meta = undefined>(action: any): action is FluxStandardAction<Payload, Meta>;
 
 /**
  * Returns `true` if `action` is FSA compliant error.
  */
-export function isError<CustomError extends Error, Meta>(action: any): action is ErrorFluxStandardAction<CustomError, Meta>;
+export function isError<CustomError extends Error, Meta = undefined>(action: any): action is ErrorFluxStandardAction<CustomError, Meta>;