IoT Events convert number to string for SNS custom payload

In IoT Events, I have setup an Event to send a message to SNS. I am attempting to use a custom payload where I can include the value in the message. The value outlined in red below is what I am trying to use and it is a Number. When I run the Analysis on my Event in the Detector GUI, it gives this warning: *Message: The payload expression "$input.CNC1Standard.vibration" isn't valid. Specify an expression that would evaluate to a string at runtime because the payload type is JSON format. *

I have looked through the documentation and cannot find the command or an example for how to convert a Number into a String for this service. I have tried both String and JSON for the Payload type, played around with quotes, escape characters, and similar. Any ideas on how to perform the conversion?