Connection String with Multisbunetfailover=true fails. States mirrored SQL Server Instance not supported

I have two SQL servers configured with an availability group. The configurations uses a dnn for it's listener. Connection tests to the configured port are successful. When we try to make a connection with multisubnetfailover=true in the connection string, we get the following error message: >Connecting to a mirrored SQL Server instance using the multiSubnetFailover connection property is not supported. This system has never been configured with a mirror. I've ran `SELECT * FROM SYS.DATABASE_MIRRORING` The results of the command was NULL. Any suggestions what I can check to resolve this?