Papertrading - Monthly Cost

I created an account for papertrading and started testing my bot. I just noticed that at the end of August about 4000 USD were subtracted from my papertrading account. Since my bot wasn’t online in the last days of August and had only 1 stock of IBM which decreased in value only 1.35 USD, I wonder why the ~4000 USD are subtracted… Is this about a monthly fee?