Bluetooth keyboard incorrect passkey to pair

Hello,

I have a several year old bluetooth keyboard that was never unpaired from a prior Macbook that is no longer in my possession. I am trying to pair it with my current Mac which is running Yosemite 10.10.5. When I go to bluetooth preferences to pair, it will recognize the keyboard but every time I enter the numeric passkey it says it is incorrect. I have tried turning on and off the keyboard, on and off bluetooth on the Mac, holding down the power button on the keyboard throughout the process as many sites have suggested, to no avail. Not sure if it's relevant but the first number on the passkey is always highlighted in blue. Any help would be greatly appreciated. Thanks