I'm trying to build a messenger bot. but I'm getting following error. the provided message is just a simple text to test the bot. I'm building this for the first time with wit.ai. please help!

error: {
message: '(#100) Message cannot be empty. A ' +
'valid attachment or text must be ' +
'provided.',
type: 'OAuthException',
code: 100,
error_subcode: 2018034,
fbtrace_id: 'A-3ZQ5XugMW6FTbM6ttwE1R'
}