SendKeys cannot run inside this application

Question

I am reading data from the serial-port in one window application which is already build by Bill Acceptor Company. Now i want to read the value from that application to my window application. I am using Sender.send key for this and it is working fine when i click on button event but i want to use timer and when i use this, I get an Exception in my .NET application saying "SendKeys cannot run inside this application because the application is not handling Windows messages".

Any help?