diff --git a/.changeset/nasty-drinks-itch.md b/.changeset/nasty-drinks-itch.md
new file mode 100644
index 00000000..450f373c
--- /dev/null
+++ b/.changeset/nasty-drinks-itch.md
@@ -0,0 +1,5 @@
+---
+"@tus/s3-store": patch
+---
+
+Fix offset being returned as `NaN` when there are no S3 parts found
diff --git a/packages/s3-store/src/index.ts b/packages/s3-store/src/index.ts
index 5009c602..e371f317 100644
--- a/packages/s3-store/src/index.ts
+++ b/packages/s3-store/src/index.ts
@@ -56,8 +56,7 @@ export type MetadataValue = {
 }
 
 function calcOffsetFromParts(parts?: Array<AWS.Part>) {
-  // @ts-expect-error not undefined
-  return parts && parts.length > 0 ? parts.reduce((a, b) => a + b.Size, 0) : 0
+  return parts && parts.length > 0 ? parts.reduce((a, b) => a + (b.Size ?? 0), 0) : 0
 }
 
 // Implementation (based on https://github.com/tus/tusd/blob/master/s3store/s3store.go)