diff --git a/tests/domain/magicplan/test_ventilation_audit.py b/tests/domain/magicplan/test_ventilation_audit.py
index 8d2542d3..6ebb3936 100644
--- a/tests/domain/magicplan/test_ventilation_audit.py
+++ b/tests/domain/magicplan/test_ventilation_audit.py
@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from pathlib import Path
+
 import openpyxl
 import pytest
 
@@ -199,3 +201,23 @@ def test_writes_zeros_when_door_has_no_ventilation() -> None:
 
     # Assert
     assert sheet["T6"].value == 0
+
+
+_TEMPLATE_PATH = (
+    Path(__file__).parent.parent.parent.parent
+    / "applications"
+    / "audit_generator"
+    / "Master Sero Template - Data Extraction.xlsx"
+)
+
+
+def test_real_template_survives_18_doors() -> None:
+    # Row 23 = DATA_START_ROW(6) + 17 — previously a merged separator that caused
+    # AttributeError: 'MergedCell' object attribute 'value' is read-only
+    wb = openpyxl.load_workbook(_TEMPLATE_PATH)
+    sheet = wb["D1 Ventilation"]
+    plan = _make_plan(num_rooms=18, num_windows_per_room=0, num_doors_per_room=1)
+
+    populate_sheet(sheet, plan)
+
+    assert sheet["R23"].value == "Room 17"