Unable to aggregate using MIN for a character column in the data model

While in the data model, I can create an aggregate function by 'dropping' a character column from a detail group into a higher level group's 'drop here for aggregate fuction' area and then go to the properties of the new column and select the MAX function to find the MAX value of the character column.  But, when I try to do the same thing to find the MIN value of the same character column I can't because only the COUNT, FIRST, LAST and MAX functions show up on the function list of the Edit Properties window.  It doesn't seem to make sense