You probably defined your BytesAvailableFCN using function handle syntax without additional arguments, like this
s.BytesAvailableFCN = @myfun();
Instead, you need to define your callback using a cell array, as explained here in the documentation. For example,
s.BytesAvailableFCN = {'myFun', handles};
handles
must already be defined and in your workspace when that line is run.