diff --git a/src/types.ts b/src/types.ts
index 86dd1c6a..260df691 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -57,7 +57,7 @@ export type ApplicationFunctionOptions = {
 export type ApplicationFunction = (
   app: Probot,
   options: ApplicationFunctionOptions
-) => void;
+) => void | Promise<void>;
 
 export type ServerOptions = {
   log?: Logger;