In redis-cli you have to write:
'zrangebylex mykey - +' for an ascending list of all members of a sorted set
'zrevrangebylex mykey + -' for a descending list

Method GetLexRange() in RedisDatabase.cs doesn't take care of a descending order in case of null-min- or null-max-values and creates command:
'zrevrangebylex mykey - +'

This always leads to an empty list