diff --git a/moto/s3/models.py b/moto/s3/models.py
index a5fddda9..6244cdd5 100644
--- a/moto/s3/models.py
+++ b/moto/s3/models.py
@@ -58,7 +58,7 @@ class FakeMultipart(object):
     def __init__(self, key_name):
         self.key_name = key_name
         self.parts = {}
-        self.id = base64.b64encode(os.urandom(43)).replace('=', '')
+        self.id = base64.b64encode(os.urandom(43)).replace('=', '').replace('+', ' ')
 
     def complete(self):
         total = bytearray()