Unknown command when using promise-ftp in node - node.js

Question

I'm trying to use a promise approach when interacting with my ftp server. I'm using the node package 'promise-ftp' v1.3.5.
The code I have is as follows:
const appFeedBuffer = Buffer.from('somedatastring', 'utf8');
var Client = require('promise-ftp');
var ftpClient = new Client();
ftpClient.connect({ host: 'myHost', user: 'me', password: 'mypassword' })
.then(() => {
return ftpClient.mkdir('newDir')
}).then(() => {
return ftpClient.put(appFeedBuffer, 'fileName')
}).then(()=> {
return ftpClient.rmdir('oldDir')
}).then(() => {
return ftpClient.rename('newDir', 'oldDir')
}).then(() => {
return ftpClient.end();
}).catch(err => {
console.log("something went wrong: " + err);
res.status(500).send('SERVER ERROR');
});
The problem is the put command - I get the error 'UNKNOWN COMMAND' which appears to come from the #icetee/ftp library, of which promise-ftp is a dependant. put is most certainly a valid command in the promise-ftp library!
Stack:
Unhandled rejection Error: Unknown command
at makeError (/Users/projects/ftpProject/node_modules/#icetee/ftp/lib/connection.js:1128:13)
at Parser.<anonymous> (/Users/projects/ftpProject/node_modules/#icetee/ftp/lib/connection.js:122:25)
at emitTwo (events.js:126:13)
at Parser.emit (events.js:214:7)
at Parser._write (/Users/projects/ftpProject/node_modules/#icetee/ftp/lib/parser.js:61:10)
at doWrite (_stream_writable.js:397:12)
at writeOrBuffer (_stream_writable.js:383:5)
at Parser.Writable.write (_stream_writable.js:290:11)
at Socket.ondata (/Users/projects/ftpProject/node_modules/#icetee/ftp/lib/connection.js:298:20)
at emitOne (events.js:116:13)
at Socket.emit (events.js:211:7)
at addChunk (_stream_readable.js:263:12)
at readableAddChunk (_stream_readable.js:250:11)
at Socket.Readable.push (_stream_readable.js:208:10)
at TCP.onread (net.js:597:20)
I can't find anyone else having this problem. What am I doing wrong?