diff --git a/hub/city_model_structure/building_demand/surface.py b/hub/city_model_structure/building_demand/surface.py
index 65f90ae3..a500c73a 100644
--- a/hub/city_model_structure/building_demand/surface.py
+++ b/hub/city_model_structure/building_demand/surface.py
@@ -42,7 +42,7 @@ class Surface:
     self._short_wave_reflectance = None
     self._long_wave_emittance = None
     self._inverse = None
-    self._associated_thermal_boundaries = []
+    self._associated_thermal_boundaries = None
     self._vegetation = None
     self._percentage_shared = None
     self._solar_collectors_area_reduction_factor = None