I'm reporting this issue on behalf of Bernt M. Johnsen.
If an indexed column is compared with a UNION query (or some other set operation), a NullPointerException is raised, as can be seen by this sequence of statements in ij:
ij> create table t(a int not null primary key, b int);
0 rows inserted/updated/deleted
ij> select * from t where a < (values 4 union values 4);
ERROR XJ001: Java exception: ': java.lang.NullPointerException'.