diff --git a/mountpoint-s3-fs/src/upload.rs b/mountpoint-s3-fs/src/upload.rs
index 1908dffbf..536380b52 100644
--- a/mountpoint-s3-fs/src/upload.rs
+++ b/mountpoint-s3-fs/src/upload.rs
@@ -116,6 +116,7 @@ where
         mem_limiter: Arc<MemoryLimiter>,
         config: UploaderConfig,
     ) -> Self {
+        assert!(config.buffer_size > 0, "buffer_size should be greater than 0 for uploads");
         Self {
             client,
             runtime,