SORT - what if there are missing values in some columns?

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http://stackoverflow.com – I want to use SORT command to sort a tabular file based on the 5th column, however, there are some missing values in the 3rd column. And I noticed that sort command does not work properly with -k5,5, but -k1,1 or -k2,2 has no problem. Is it because of the missing values? (HowTos)