diff --git a/stringify.ts b/stringify.ts
index 10b3a5d..b8b7836 100644
--- a/stringify.ts
+++ b/stringify.ts
@@ -467,7 +467,7 @@ function buildParts(lines: LineArray, parts: PartialSegment[]) {
   return hint;
 }
 
-function stringify(playlist: MasterPlaylist | MediaPlaylist, postProcess: PostProcess | undefined): string {
+function stringify(playlist: MasterPlaylist | MediaPlaylist, postProcess?: PostProcess): string {
   utils.PARAMCHECK(playlist);
   utils.ASSERT('Not a playlist', playlist.type === 'playlist');
   const lines = new LineArray(playlist.uri);
diff --git a/types.ts b/types.ts
index 6c278a1..1151625 100644
--- a/types.ts
+++ b/types.ts
@@ -520,6 +520,6 @@ export type TagParam =
 export type UserAttribute = number | string | Uint8Array;
 
 export type PostProcess = {
-  segmentProcessor: ((lines: string[], start: number, end: number, segment: Segment, i: number) => undefined) | undefined;
-  variantProcessor: ((lines: string[], start: number, end: number, variant: Variant, i: number) => undefined) | undefined;
+  segmentProcessor: ((lines: string[], start: number, end: number, segment: Segment, i: number) => void) | undefined;
+  variantProcessor: ((lines: string[], start: number, end: number, variant: Variant, i: number) => void) | undefined;
 };