diff --git a/aws-s3/s3.ml b/aws-s3/s3.ml
index ad02512..bf0ff0b 100644
--- a/aws-s3/s3.ml
+++ b/aws-s3/s3.ml
@@ -519,7 +519,7 @@ module Make(Io : Types.Io) = struct
         |> fun etag -> unquote etag
       in
       t.parts <- { etag; part_number } :: t.parts;
-      Deferred.return (Ok ())
+      Deferred.return (Ok etag)
 
     (** Specify a part to be a file on s3.
         [range] can be used to only include a part of the s3 file
@@ -616,7 +616,7 @@ module Make(Io : Types.Io) = struct
           |> fun etag -> String.sub ~pos:1 ~len:(String.length etag - 2) etag
         in
         t.parts <- { etag; part_number } :: t.parts;
-        Deferred.return (Ok ())
+        Deferred.return (Ok etag)
     end
 
   end
diff --git a/aws-s3/s3.mli b/aws-s3/s3.mli
index d153d63..051736a 100644
--- a/aws-s3/s3.mli
+++ b/aws-s3/s3.mli
@@ -216,7 +216,7 @@ module Make(Io : Types.Io) : sig
        ?expect:bool ->
        data:string ->
        unit ->
-       unit result) command
+       etag result) command
 
     (** Specify a part as a copy of an existing object in S3. *)
     val copy_part :
@@ -255,7 +255,7 @@ module Make(Io : Types.Io) : sig
          length:int ->
          chunk_size:int ->
          unit ->
-         unit result) command
+         etag result) command
     end
   end
 
diff --git a/cli/aws.ml b/cli/aws.ml
index e288060..9cd67ff 100644
--- a/cli/aws.ml
+++ b/cli/aws.ml
@@ -95,15 +95,18 @@ module Make(Io : Aws_s3.Types.Io) = struct
     | (false, false) -> failwith "Use cp(1)"
 
   let rec upload_parts t endpoint ~retries ~expect ~credentials ?(offset=0) ~total ?(part_number=1) ?chunk_size src =
+    let ignore_result = Result.map ignore in
     let f ~size ~endpoint ()=
       match chunk_size with
       | None ->
         let data = read_file ~pos:offset ~len:size src in
-        S3.Multipart_upload.upload_part ~endpoint ~expect ~credentials t ~part_number ~data ()
+        S3.Multipart_upload.upload_part ~endpoint ~expect ~credentials t ~part_number ~data () >>|
+        ignore_result
       | Some chunk_size ->
         (* Create a reader for this section *)
         let reader = file_reader ~pos:offset ~len:size src in
-        S3.Multipart_upload.Stream.upload_part ~endpoint ~expect ~credentials t ~part_number ~data:reader ~chunk_size ~length:size ()
+        S3.Multipart_upload.Stream.upload_part ~endpoint ~expect ~credentials t ~part_number ~data:reader ~chunk_size ~length:size () >>|
+        ignore_result
     in
     match (total - offset) with
     | 0 -> []