diff --git a/index.d.ts b/index.d.ts
index 6cafda6..c9fea9f 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -47,16 +47,24 @@ import * as closeWithGrace from 'close-with-grace'
   })
  */
 declare function closeWithGrace (
-  opts:
-    | closeWithGrace.CloseWithGraceAsyncCallback
-    | closeWithGrace.CloseWithGraceCallback
-    | closeWithGrace.Options
+  fn: closeWithGrace.CloseWithGraceAsyncCallback
+): CloseWithGraceReturn
+declare function closeWithGrace (
+  fn: closeWithGrace.CloseWithGraceCallback
+): CloseWithGraceReturn
+declare function closeWithGrace (
+  opts: closeWithGrace.Options,
+  fn: closeWithGrace.CloseWithGraceAsyncCallback
 ): CloseWithGraceReturn
 declare function closeWithGrace (
   opts: closeWithGrace.Options,
-  fn?:
-    | closeWithGrace.CloseWithGraceAsyncCallback
-    | closeWithGrace.CloseWithGraceCallback
+  fn: closeWithGrace.CloseWithGraceCallback
+): CloseWithGraceReturn
+declare function closeWithGrace (
+  fn: closeWithGrace.CloseWithGraceAsyncCallback
+): CloseWithGraceReturn
+declare function closeWithGrace (
+  fn: closeWithGrace.CloseWithGraceCallback
 ): CloseWithGraceReturn
 
-export = closeWithGrace
\ No newline at end of file
+export = closeWithGrace
diff --git a/index.test-d.ts b/index.test-d.ts
index 9888749..3f86ef3 100644
--- a/index.test-d.ts
+++ b/index.test-d.ts
@@ -71,4 +71,12 @@ expectType<{
 expectType<{
   close: () => void
   uninstall: () => void
-}>(closeWithGrace({ delay: 100 }, AllCallback))
\ No newline at end of file
+}>(closeWithGrace({ delay: 100 }, AllCallback))
+
+closeWithGrace({ delay: 100 }, async function ({ err }) {
+  expectType<Error | undefined>(err)
+})
+
+closeWithGrace(async function ({ err }) {
+  expectType<Error | undefined>(err)
+})