Google Earth Engine - use reduce(ee.Reducer.max())

Question

I tried to used reduce(ee.Reducer.max()) and reduce(ee.Reducer.min())in the follow function, but doesn't work very well. Max and min return the same value. How can I solve this issue? Thank you.

var SM = function(image) {

  var FE = image.expression('((SM-SMmin)/(SMmax-SMmin))',{
        'SM':image.select('susm'),
        'SMmax':image.select('susm').reduce(ee.Reducer.max()),
        'SMmin':image.select('susm').reduce(ee.Reducer.min()),
  
        })

  return FE;
};