How Do I Enable the M140 Heating Bed Option?

I am using Marlin 1.1.9. I am trying to heat my bad using the 'M140 S60' command but it's not working. When I enter this in Pronterface I get:

Setting bed temperature to 60.000000 degrees Celsius.
echo:Unknown command: "M140 S60.0"

Does anyone know how to enable this in the firmware?