postgresql 10日期范围限制

我想在postgresql 10中设置一个日期范围约束。在postgresql 9.6中,这起到了作用:

CREATE TABLE project_lines (
  id SERIAL PRIMARY KEY,
  project_id INTEGER NOT NULL REFERENCES projects(id),
  description VARCHAR(200) NOT NULL,
  start_time TIMESTAMP NOT NULL,
  end_time TIMESTAMP CHECK(end_time > start_time),
  created_at TIMESTAMP NOT NULL DEFAULT NOW(),
  CONSTRAINT overlapping_times EXCLUDE USING GIST(
    project_id WITH =,
    tstzrange(start_time, COALESCE(end_time, 'infinity')) WITH &&
  )
);

但在postgresql 10中,我遇到了以下错误:
functions in index expression must be marked IMMUTABLE

如何使此约束生效?