Invalid Serial error

 
I keep getting this error from my MT4 terminal saying I have entered an "invalid Serial". I use Oanda as my broker and the expert Im using has worked fine on other accounts. Any suggestions?
 

If this message only occurs when running your EA, I suggest you contact the supplier.

Some commercial EAs allow unlimited Demo accounts but need registering of live accounts.

If your EA is proprietary, then maybe it's to do with (not) registering your trading account.