Assign an automatic track width to a net class?

v9.0.2
When I draw track I’d like the PCB Editor to automatically draw a particular track width according to its net class. In ‘Set Track and Via Properties’ the option to set the track width is locked to ‘-- leave unchanged --’. What needs to be done to allow a width to be assigned to that net class?

KICAD - SET TRACK AND VIA PROPERTIES