diff --git a/apps/predbat/axle.py b/apps/predbat/axle.py
index e3357d790..2d93dda03 100644
--- a/apps/predbat/axle.py
+++ b/apps/predbat/axle.py
@@ -29,10 +29,14 @@ class AxleAPI(ComponentBase):
 
     def initialize(self, api_key, pence_per_kwh, automatic):
         """Initialize the AxleAPI component"""
+        if not isinstance(api_key, str) or not api_key:
+            self.log("Error: AxleAPI: axle_api_key is missing or invalid, you must set it to a string (not a list or number). Axle Energy integration will not function correctly.")
+            api_key = None
         self.api_key = api_key
         self.pence_per_kwh = pence_per_kwh
         self.automatic = automatic
         self.failures_total = 0
+        self.history_loaded = False
         self.event_history = []  # List of past events
         self.current_event = {  # Current event
             "start_time": None,
@@ -40,7 +44,7 @@ def initialize(self, api_key, pence_per_kwh, automatic):
             "import_export": None,
             "pence_per_kwh": None,
         }
-        self.updated_at: None  # Last updated moved out to separate attribute to not pollute triggering on change of current_event
+        self.updated_at = None  # Last updated moved out to separate attribute to not pollute triggering on change of current_event
 
     def load_event_history(self):
         """
@@ -185,6 +189,11 @@ async def fetch_axle_event(self):
         """
         Fetch the latest VPP event from Axle Energy API
         """
+        if not self.api_key:
+            self.log("Error: AxleAPI: Cannot fetch event - axle_api_key is not set or invalid. Please check your apps.yaml configuration.")
+            self.failures_total += 1
+            return
+
         self.log("AxleAPI: Fetching latest VPP event data")
 
         url = "https://api.axle.energy/vpp/home-assistant/event"
diff --git a/apps/predbat/tests/test_axle.py b/apps/predbat/tests/test_axle.py
index 44982d967..4ee595730 100644
--- a/apps/predbat/tests/test_axle.py
+++ b/apps/predbat/tests/test_axle.py
@@ -114,6 +114,7 @@ def test_axle(my_predbat=None):
     # Sub-test registry - each entry is (key, function, description)
     sub_tests = [
         ("initialization", _test_axle_initialization, "Axle API initialization"),
+        ("list_api_key", _test_axle_list_api_key_validation, "List API key validation and error logging"),
         ("active_event", _test_axle_fetch_with_active_event, "Fetch with active event"),
         ("duplicate_event", _test_axle_duplicate_event_detection, "Duplicate event detection"),
         ("notify_config", _test_axle_fetch_with_notify_config, "Notification config control"),
@@ -174,6 +175,7 @@ def _test_axle_initialization(my_predbat=None):
     assert axle.pence_per_kwh == 150, "Pence per kWh not set correctly"
     assert axle.failures_total == 0, "Failures should be 0 on init"
     assert axle.last_updated_timestamp is None, "Last updated should be None on init"
+    assert axle.updated_at is None, "updated_at should be None on init"
     assert axle.current_event["start_time"] is None, "Current event should be None on init"
     assert axle.event_history == [], "Event history should be empty on init"
     assert axle.history_loaded is False, "History should not be loaded on init"
@@ -182,6 +184,26 @@ def _test_axle_initialization(my_predbat=None):
     return False
 
 
+def _test_axle_list_api_key_validation(my_predbat=None):
+    """Test that a list-type API key (incorrect YAML format) is handled with clear error logging"""
+    print("Test: Axle API list API key validation")
+
+    # Case 1: api_key is a non-empty list (most common misconfiguration)
+    axle = MockAxleAPI()
+    axle.initialize(api_key=["correct_key_as_list"], pence_per_kwh=100, automatic=False)
+
+    assert axle.api_key is None, "Should failed to allow a list as an API key"
+
+    # Case 2: api_key is an empty list
+    axle2 = MockAxleAPI()
+    axle2.initialize(api_key=[], pence_per_kwh=100, automatic=False)
+
+    assert axle2.api_key is None, "Empty list should result in None api_key"
+
+    print("  ✓ List API key validation works correctly")
+    return False
+
+
 def _test_axle_fetch_with_active_event(my_predbat=None):
     """Test fetching an active VPP event from API"""
     print("Test: Axle API fetch with active event")