ValidationError: user: Cast to Embedded failed for value "[]" - mongodb

Question

I am trying to insert a new message into the database using expressjs and mongoose.
But I'm getting a ValidationError in the process. The following is printed at the console.
error while storing message ::: ValidationError: user: Cast to
Embedded failed for value "[]" at path "user"
The code is as follows:
socketAPI.io.on('connection', function(socket){
socket.on('chat-sent', function(param) {
ChatUser.find({username : param.username}).then(async (usr)=>{
if(usr){
try{
var mess = new GroupMessage();
mess.group = (await Group.findOne({name : groupName}));
mess.message = param.message;
mess.user = usr;
mess.save((err)=>{
if(err){
console.log('error while storing message ::: '+err);
}
else{
console.log('message succesfully stored');
}
});
}catch(err){
console.log('error collecting group ::: '+err);
}
}
});
socket.emit('group-message-handled', { user : param.username, message : param.message});
});
});
The insertion code makes use of the following models:
var userSchema = new mgoose.Schema({
username : {
type : String,
required : true,
unique : true,
maxlength : [15, 'Too long username. Max 15 allowed']
},
name : String
});
var groupSchema = new mgoose.Schema({
name : {
type : String,
required : true,
unique : true
},
categories : [String]
});
var groupMessageSchema = new mgoose.Schema({
user : userSchema,
group : groupSchema,
message : {
type : String,
required : true,
minlength : 1
}
});
var ChatUser = mgoose.model('ChatUser', userSchema);
var Group = mgoose.model('Group', groupSchema);
var GroupMessage = mgoose.model('GroupMessage', groupMessageSchema);
What causes this error and how can I fix it?