diff --git a/backend/categorisation/tests/test_prioritised_plan_selected.py b/backend/categorisation/tests/test_prioritised_plan_selected.py
index 7544eb9c..e2af6a63 100644
--- a/backend/categorisation/tests/test_prioritised_plan_selected.py
+++ b/backend/categorisation/tests/test_prioritised_plan_selected.py
@@ -1,5 +1,5 @@
 from datetime import datetime
-from typing import List
+from typing import List, Optional
 import pytest
 
 from backend.app.domain.classes.plan import Plan
@@ -16,7 +16,7 @@ def created_at_datetime() -> datetime:
 
 
 def make_plan_record(
-    created_at: datetime, default: bool, cost_of_works: float = 500.0
+    created_at: datetime, default: bool, cost_of_works: Optional[float] = 500.0
 ) -> PlanRecord:
     return PlanRecord(
         property_id=1,
@@ -43,7 +43,10 @@ def make_scenario(name: str, created_at: datetime, is_default: bool) -> Scenario
 
 
 def make_plan(
-    created_at: datetime, default: bool, cost_of_works: float = 500.0, name: str = ""
+    created_at: datetime,
+    default: bool,
+    cost_of_works: Optional[float] = 500.0,
+    name: str = "",
 ) -> Plan:
     scenario = make_scenario(name, created_at, default)
     plan_id = 1 if default else 2
@@ -113,3 +116,24 @@ def test_all_plans_zero_cost__highest_priority_returned(
 
     # assert
     assert actual_default_plan.id == expected_default_plan_id
+
+
+def test_all_plans_null_cost__highest_priority_returned(
+    created_at_datetime: datetime,
+) -> None:
+    # arrange
+    epc_c_plan = make_plan(created_at_datetime, True, cost_of_works=None, name="EPC C")
+    minor_works_plan = make_plan(
+        created_at_datetime, False, cost_of_works=None, name="EPC C - Minor Works"
+    )
+    scenario_priority_order: List[int] = [4, 3]
+    expected_default_plan_id = 2
+
+    # act
+    actual_default_plan = choose_cheapest_relevant_plan(
+        plans=[epc_c_plan, minor_works_plan],
+        scenario_priority_order=scenario_priority_order,
+    )
+
+    # assert
+    assert actual_default_plan.id == expected_default_plan_id