diff --git a/chartpress.py b/chartpress.py
index <HASH>..<HASH> 100755
--- a/chartpress.py
+++ b/chartpress.py
@@ -266,7 +266,7 @@ def main():
         help='Use this tag for images & charts')
     argparser.add_argument('--extra-message', default='',
         help='extra message to add to the commit message when publishing charts')
-    argparser.add_argument('--set-prefix', default='',
+    argparser.add_argument('--set-prefix', default=None,
         help='override image prefix with this value')
 
     args = argparser.parse_args()
@@ -276,7 +276,7 @@ def main():
 
     for chart in config['charts']:
         if 'images' in chart:
-            image_prefix = args.set_prefix if args.set_prefix != '' else chart['imagePrefix']
+            image_prefix = args.set_prefix if args.set_prefix is not None else chart['imagePrefix']
             value_mods = build_images(
                 prefix=image_prefix,
                 images=chart['images'],
Use None as default for --set-prefix