diff --git a/ova-compose/ova-compose.py b/ova-compose/ova-compose.py
index 1eee91c..02486c6 100755
--- a/ova-compose/ova-compose.py
+++ b/ova-compose/ova-compose.py
@@ -644,8 +644,8 @@ def __init__(self, **kwargs):
             for k, v in props.items():
                 self.properties.append(OVFProperty(k, **v))
 
-        self.transports = kwargs.get('transports', None)
-        self.categories = kwargs.get('categories', None)
+        self.transports = kwargs.get('transports', [])
+        self.categories = kwargs.get('categories', {})
 
         # if a property references a non-existing category it will be dropped
         for prop in self.properties:
@@ -867,6 +867,8 @@ def from_dict(cls, config):
         annotation = None
         eula = None
 
+        assert 'hardware' in config, "config needs a 'hardware' section"
+
         hardware = config['hardware']
         for hw_id, hw in hardware.items():
             if isinstance(hw, dict):