org.apache.tajo.algebra.FunctionExpr cannot be cast to org.apache.tajo.algebra.ColumnReferenceExpr

SELECT
                col1, col2, col3, 
                CASE
                  WHEN split_part(split_part(col4,'id=', 2), '&', 1) IS NOT NULL THEN ''
                  ELSE
                    CASE
                      WHEN col4 like '/%' THEN split_part(col4,'/', 2)
                      ELSE split_part(col4,'/', 1)
                    END
                END AS col4, split_part(split_part(col4,'id=', 2), '&', 1) AS id
        FROM table1
        WHERE tmp='asdf';

org.apache.tajo.algebra.FunctionExpr cannot be cast to org.apache.tajo.algebra.ColumnReferenceExpr