diff --git a/aioboto3/s3/inject.py b/aioboto3/s3/inject.py
index 1c842da..4f512b6 100644
--- a/aioboto3/s3/inject.py
+++ b/aioboto3/s3/inject.py
@@ -265,7 +265,7 @@ async def file_reader() -> None:
         eof = False
         while not eof:
             part += 1
-            multipart_payload = b''
+            multipart_payload = bytearray()
             loop_counter = 0
             while len(multipart_payload) < multipart_chunksize:
                 try:
@@ -284,7 +284,7 @@ async def file_reader() -> None:
 
                     # shortcircuit upload logic
                     eof = True
-                    multipart_payload = b''
+                    multipart_payload = bytearray()
                     break
 
                 if data == b'' and loop_counter > 0:  # End of file, handles uploading empty files