diff --git a/pisa/api.py b/pisa/api.py
index 131ce43..88cccf8 100644
--- a/pisa/api.py
+++ b/pisa/api.py
@@ -77,6 +77,9 @@ def get_appointment():
     response = []
 
     # ToDo: #15-add-system-monitor
+    if not isinstance(locator, str) or len(locator) != 64:
+        response.append({"locator": locator, "status": "not_found"})
+        return jsonify(response)
 
     locator_map = watcher.db_manager.load_locator_map(locator)