diff --git a/aviary/core/aviary_group.py b/aviary/core/aviary_group.py
index ec51f187f..1174c7304 100644
--- a/aviary/core/aviary_group.py
+++ b/aviary/core/aviary_group.py
@@ -69,6 +69,7 @@ def __init__(self, verbosity=None, **kwargs):
         self.engine_models = []
         self.regular_phases = []
         self.reserve_phases = []
+        self.subsystems = []
 
         self.aviary_inputs = None
         self.meta_data = None