NullPointerException on unique constraint violation with unique index

Given the below DDL, violations of the unique constraint (an example follows the DDL) specified on table t1 will result in a NullPointerException that terminates the active connection. The root cause appears to be related to the unique indexes; removing them results in the correct constraint violation.

CREATE TABLE s.t0 (t0_id INTEGER NOT NULL, value VARCHAR(75) NOT NULL);
ALTER TABLE s.t0 ADD CONSTRAINT pk_1 PRIMARY KEY (t0_id);

CREATE TABLE s.t1 (t1_id INTEGER NOT NULL, t0_id INTEGER NOT NULL, value VARCHAR(75) NOT NULL);
CREATE UNIQUE INDEX s.ui_1 ON s.t1 (t1_id);

ALTER TABLE s.t1 ADD CONSTRAINT pk_2 PRIMARY KEY (t1_id);
ALTER TABLE s.t1 ADD CONSTRAINT fk_1 FOREIGN KEY (t0_id) REFERENCES s.t0 (t0_id) ON DELETE CASCADE;
CREATE UNIQUE INDEX s.ui_2 ON s.t1 (t0_id, value);
ALTER TABLE s.t1 ADD CONSTRAINT uc_1 UNIQUE (t0_id, value);

Minimal DML to trigger the constraint violation:

INSERT INTO s.t0 VALUES (0, 'foobar');
INSERT INTO s.t1 VALUES(0, 0, 'Test');
INSERT INTO s.t1 VALUES(1, 0, 'Test');