cqlsh:foo> CREATE TABLE users (
       ...   userid text PRIMARY KEY,
       ...   first_name text,
       ...   last_name text,
       ...   emails set<text>,
       ...   top_scores list<int>,
       ...   todo map<timestamp, text>
       ... );
cqlsh:foo> create custom index bar on foo.users (userid, last_name);
InvalidRequest: Error from server: code=2200 [Invalid query] message="Cannot create secondary index on partition key column userid"