diff --git a/infrastructure/chatgpt/chatgpt_property_type_classifier.py b/infrastructure/chatgpt/chatgpt_property_type_classifier.py
index d4f0c060..75ec1556 100644
--- a/infrastructure/chatgpt/chatgpt_property_type_classifier.py
+++ b/infrastructure/chatgpt/chatgpt_property_type_classifier.py
@@ -33,6 +33,14 @@ class ChatGptPropertyTypeClassifier(PropertyTypeClassifier):
         )
         raw: dict[str, Any] = json.loads(reply)
         return {
-            description: PropertyType(raw[description])
+            description: self._to_property_type(raw[description])
             for description in descriptions
         }
+
+    @staticmethod
+    def _to_property_type(value: Any) -> PropertyType:
+        """Map a reply value to a PropertyType, defaulting to UNKNOWN."""
+        try:
+            return PropertyType(value)
+        except ValueError:
+            return PropertyType.UNKNOWN
diff --git a/tests/infrastructure/chatgpt/test_chatgpt_property_type_classifier.py b/tests/infrastructure/chatgpt/test_chatgpt_property_type_classifier.py
index 8c697eb2..d4801154 100644
--- a/tests/infrastructure/chatgpt/test_chatgpt_property_type_classifier.py
+++ b/tests/infrastructure/chatgpt/test_chatgpt_property_type_classifier.py
@@ -31,3 +31,15 @@ def test_classifies_description_into_property_type() -> None:
 
     # Assert
     assert result == {"semi-detached": PropertyType.HOUSE}
+
+
+def test_unrecognised_category_maps_to_unknown() -> None:
+    # Arrange
+    chat_gpt = _FakeChatGPT(reply='{"garden shed": "Shed"}')
+    classifier = ChatGptPropertyTypeClassifier(chat_gpt)
+
+    # Act
+    result = classifier.classify({"garden shed"})
+
+    # Assert
+    assert result == {"garden shed": PropertyType.UNKNOWN}