diff --git a/formats/aci.py b/formats/aci.py
index dd9bf4ff9..b5c40effd 100644
--- a/formats/aci.py
+++ b/formats/aci.py
@@ -178,7 +178,7 @@ class ACIImage(TarImageFormatter):
         "eventHandlers": [],
         "workingDirectory": config.get('WorkingDir', '') or '/',
         "environment": [{"name": key, "value": value}
-                        for (key, value) in [e.split('=') for e in config.get('Env', [])]],
+                        for (key, value) in [e.split('=') for e in config.get('Env', []) or []]],
         "isolators": ACIImage._build_isolators(config),
         "mountPoints": ACIImage._build_volumes(config),
         "ports": ACIImage._build_ports(config),