diff --git a/cdisutils/storage3.py b/cdisutils/storage3.py
index 73d1a7e..c52fe5f 100644
--- a/cdisutils/storage3.py
+++ b/cdisutils/storage3.py
@@ -18,11 +18,11 @@
 import sys
 import time
 from builtins import next, object, str, zip
-from io import StringIO as BIO
 from urllib.parse import urlparse
 
 import boto3
 import urllib3
+import six
 from botocore.exceptions import ClientError
 
 from .log import get_logger
@@ -345,7 +345,7 @@ def create_multipart_upload(self, src_url=None, dst_url=None):
 
         multipart_info["dst_info"] = self.parse_url(url=dst_url)
         multipart_info["src_info"] = self.parse_url(url=src_url)
-        multipart_info["stream_buffer"] = BIO()
+        multipart_info["stream_buffer"] = six.BytesIO()
         multipart_info["mp_chunk_size"] = self.mp_chunk_size
         multipart_info["download_chunk_size"] = self.chunk_size
         multipart_info["cur_size"] = 0
@@ -404,7 +404,7 @@ def upload_multipart_chunk(self, mp_info):
         else:
             mp_info["cur_size"] = 0
             mp_info["stream_buffer"].close()
-            mp_info["stream_buffer"] = BIO()
+            mp_info["stream_buffer"] = six.BytesIO()
             mp_info_part = {
                 "ETag": result["ETag"],
                 "PartNumber": mp_info["chunk_index"],
@@ -424,10 +424,10 @@ def copy_multipart_file(
         multipart between object stores
         """
 
-        if isinstance(src_info, str):
+        if isinstance(src_info, six.string_types):
             src_url = src_info
             src_info = self.parse_url(url=src_url)
-        if isinstance(dst_info, str):
+        if isinstance(dst_info, six.string_types):
             dst_url = dst_info
             dst_info = self.parse_url(url=dst_url)