diff --git a/aiohttp/multipart.py b/aiohttp/multipart.py
index 9d5e5d27b84..4d4eee7887f 100644
--- a/aiohttp/multipart.py
+++ b/aiohttp/multipart.py
@@ -322,8 +322,8 @@ async def read(self, *, decode: bool = False) -> bytes:
                 decoded_data.extend(d)
                 if len(decoded_data) > self._client_max_size:
                     raise self._max_size_error_cls(self._client_max_size)
-            return decoded_data
-        return data
+            return bytes(decoded_data)
+        return bytes(data)
 
     async def read_chunk(self, size: int = chunk_size) -> bytes:
         """Reads body part content chunk of the specified size.