diff --git a/src/volume.ts b/src/volume.ts
index b3b5c5b8..500c91ab 100644
--- a/src/volume.ts
+++ b/src/volume.ts
@@ -193,7 +193,7 @@ export type TFlagsCopy =
   | typeof constants.COPYFILE_FICLONE
   | typeof constants.COPYFILE_FICLONE_FORCE;
 
-export function flagsToNumber(flags: TFlags): number {
+export function flagsToNumber(flags: TFlags | undefined): number {
   if (typeof flags === 'number') return flags;
 
   if (typeof flags === 'string') {