diff --git a/website/web/__init__.py b/website/web/__init__.py
index e8e2431..4d4de83 100644
--- a/website/web/__init__.py
+++ b/website/web/__init__.py
@@ -633,8 +633,12 @@ def monitor(tree_uuid: str):
     collection: str = request.form['collection'] if request.form.get('collection') else ''
     frequency: str = request.form['frequency'] if request.form.get('frequency') else 'daily'
     cache = lookyloo.capture_cache(tree_uuid)
-    monitoring_uuid = lookyloo.monitoring.monitor({'url': cache.url}, frequency=frequency, collection=collection)
-    flash(f"Sent to monitoring: {monitoring_uuid}", 'success')
+    if cache:
+        monitoring_uuid = lookyloo.monitoring.monitor({'url': cache.url, 'user_agent': cache.user_agent},
+                                                      frequency=frequency, collection=collection)
+        flash(f"Sent to monitoring: {monitoring_uuid}", 'success')
+    else:
+        flash(f"Unable to send to monitoring, uuid {tree_uuid} not found in cache.", 'error')
     return redirect(url_for('tree', tree_uuid=tree_uuid))
 
 
diff --git a/website/web/genericapi.py b/website/web/genericapi.py
index 9da294b..b2b726a 100644
--- a/website/web/genericapi.py
+++ b/website/web/genericapi.py
@@ -462,9 +462,12 @@ class CompareCaptures(Resource):
             result = comparator.compare_captures(left_uuid, right_uuid)
         except MissingUUID as e:
             # UUID non-existent, or capture still ongoing.
-            status_left = lookyloo.get_capture_status(left_uuid)
-            status_right = lookyloo.get_capture_status(right_uuid)
-            return {'error': e, 'details': {left_uuid: status_left, right_uuid: status_right}}
+            if left_uuid and right_uuid:
+                status_left = lookyloo.get_capture_status(left_uuid)
+                status_right = lookyloo.get_capture_status(right_uuid)
+                return {'error': e, 'details': {left_uuid: status_left, right_uuid: status_right}}
+            else:
+                return {'error': e, 'details': 'Invalid request (left/right UUIDs missing.)'}
         return result