diff --git a/kolla/cmd/build.py b/kolla/cmd/build.py
index 243ec77f74..b907673664 100755
--- a/kolla/cmd/build.py
+++ b/kolla/cmd/build.py
@@ -299,7 +299,7 @@ class KollaWorker(object):
                         images_to_process.append(image)
                         added = True
         else:
-            images_to_process = self.images
+            images_to_process = list(self.images)
 
         self.tiers = list()
         while images_to_process: