diff --git a/formats/aci.py b/formats/aci.py
index 83ec9f3b9..c24f691bd 100644
--- a/formats/aci.py
+++ b/formats/aci.py
@@ -184,7 +184,7 @@ class ACIImage(TarImageFormatter):
     hostname = hostname.split(':', 1)[0]
 
     # Calculate the environment variables.
-    docker_env_vars = config.get('Env', [])
+    docker_env_vars = config.get('Env', []) or []
     env_vars = []
     for var in docker_env_vars:
       pieces = var.split('=')