diff --git a/app.py b/app.py
index 135c71a12c3bf78783daacf77130f57e5bca85cb..a29922a165f4610f193758bc2a672b50f9c2f10f 100644
--- a/app.py
+++ b/app.py
@@ -22,7 +22,7 @@ def ping():
 
 
 @app.route("/send", methods=["GET", "POST"])
-def send_message():
+def send_message():  # noqa: MC0001
     """Send a notification."""
     if request.method == "POST":
         # Needs to be called before accessing other request parameters,
@@ -39,6 +39,9 @@ def send_message():
     else:
         message = request.args["message"]
 
+    if message is None or message.strip() == "":
+        raise RuntimeError("No message or message is empty.")
+
     for c in client.containers.list():  # pylint: disable=invalid-name
         if c.name == CONTAINER_NAME:
             container = c