Log in issues with bluetooth keyboard on iMac since El Capitan install

Hi, Since installing El Capitan on iMac, after being logged in and returning to the log in screen after a while, the password entry field fills up itself as if a key is stuck on the bluetooth keyboard which is not. Only happens at login. I have to turn of the keyboard to stop the characters from entering. Once that has been done I am able to enter password as normal. Has anyone else had this issue?