added pushoverKeyHasWrongLength() check

src/Exceptions/CouldNotSendNotification.php

@@ -29,4 +29,15 @@ public static function serviceRespondedWithAnError(ResponseInterface $response,
 
         return new static($exceptionMessage, $statusCode);
     }
+
+    public static function pushoverKeyHasWrongLength($notifiable)
+    {
+        $exceptionMessage = sprintf(
+            "Pushover key has wrong length for notifiable '%s' with id '%s'. It needs to be 30 characters long.",
+            get_class($notifiable),
+            $notifiable->getKey()
+        );
+
+        return new static($exceptionMessage);
+    }
 }

src/PushoverChannel.php

@@ -5,6 +5,7 @@
 use Illuminate\Contracts\Events\Dispatcher;
 use Illuminate\Notifications\Events\NotificationFailed;
 use Illuminate\Notifications\Notification;
+use NotificationChannels\Pushover\Exceptions\CouldNotSendNotification;
 use NotificationChannels\Pushover\Exceptions\ServiceCommunicationError;
 
 class PushoverChannel
@@ -41,6 +42,12 @@ public function send($notifiable, Notification $notification)
         }
 
         if (is_string($pushoverReceiver)) {
+            // From https://pushover.net/api:
+            // "User and group identifiers are 30 characters long, ..."
+            if (strlen($pushoverReceiver) !== 30) {
+                throw CouldNotSendNotification::pushoverKeyHasWrongLength($notifiable);
+            }
+
             $pushoverReceiver = PushoverReceiver::withUserKey($pushoverReceiver);
         }