diff --git a/nodepool/zk.py b/nodepool/zk.py
index 6e123ccde..99ef4203f 100755
--- a/nodepool/zk.py
+++ b/nodepool/zk.py
@@ -214,7 +214,7 @@ class ImageBuild(BaseModel):
 
     @property
     def formats(self):
-        return self._formats
+        return sorted(self._formats)
 
     @formats.setter
     def formats(self, value):