diff --git a/watertap/unit_models/zero_order/feed_zo.py b/watertap/unit_models/zero_order/feed_zo.py
index 05129243b1..768ec8c9ab 100644
--- a/watertap/unit_models/zero_order/feed_zo.py
+++ b/watertap/unit_models/zero_order/feed_zo.py
@@ -195,3 +195,8 @@ def initialize_build(
                 f"{self.name} failed to initialize successfully. Please check "
                 f"the output logs for more information."
             )
+
+    # no costing method
+    @property
+    def default_costing_method(self):
+        return lambda *args, **kwargs: None
diff --git a/watertap/unit_models/zero_order/gas_sparged_membrane_zo.py b/watertap/unit_models/zero_order/gas_sparged_membrane_zo.py
index d63968ae8e..263c0fc42b 100644
--- a/watertap/unit_models/zero_order/gas_sparged_membrane_zo.py
+++ b/watertap/unit_models/zero_order/gas_sparged_membrane_zo.py
@@ -210,6 +210,11 @@ def solute_treated_equation(b, t, j):
         self._Q = Reference(self.properties_in[:].flow_vol)
         pump_electricity(self, self._Q)
 
+    # no costing method
+    @property
+    def default_costing_method(self):
+        return lambda *args, **kwargs: None
+
 
 def calculate_scaling_factors_gas_extraction(self):
     # Get default scale factors and do calculations from base classes