diff --git a/tools/build.py b/tools/build.py
index e0309c270d..ac42cac2b7 100755
--- a/tools/build.py
+++ b/tools/build.py
@@ -89,7 +89,7 @@ class WorkerThread(Thread):
             image['status'] = "parent_error"
             return
 
-        if image['source']:
+        if 'source' in image:
             self.process_source(image['source'], image['path'])
 
         # Pull the latest image for the base distro only