diff --git a/cluster/clustercfg/clustercfg.py b/cluster/clustercfg/clustercfg.py
index ea15df8..7024f4a 100644
--- a/cluster/clustercfg/clustercfg.py
+++ b/cluster/clustercfg/clustercfg.py
@@ -42,7 +42,7 @@
 def pki_config(key, fqdn):
     machine_name = fqdn.split('.')[0]
     raw = subprocess.check_output([
-        'nix', 'eval', '--raw',
+        'nix', 'eval', '--raw', '--impure', '--expr',
         '( ((import ' + local_root + '/cluster/nix/defs-cluster-k0.nix ) "' + machine_name + '").pki.' + key + '.json )',
     ])
     return json.loads(raw)