Unquoted expressions are not supported

Liberty specification provides examples like the following (Example 1-6):

input_voltage(CMOS) {
  vil : 0.3 * VDD ;
  vih : 0.7 * VDD ;
  vimin : -0.5 ;
  vimax : VDD + 0.5 ;
}

input_voltage(TTL_5V) {
  vil : 0.8 ;
  vih : 2.0 ;
  vimin : -0.5 ;
  vimax : VDD + 0.5 ;
}

The issue is expressions are not enquoted, resulting in parsing error.