diff --git a/src/test/regress/expected/for_portion_of.out b/src/test/regress/expected/for_portion_of.out
index 271282c2d3b..0e217f104ef 100644
--- a/src/test/regress/expected/for_portion_of.out
+++ b/src/test/regress/expected/for_portion_of.out
@@ -1843,6 +1843,53 @@ SELECT * FROM for_portion_of_test ORDER BY valid_at;
 DROP FUNCTION fpo_append_name_suffix CASCADE;
 NOTICE:  drop cascades to trigger fpo_before_insert_row on table for_portion_of_test
 DROP TABLE for_portion_of_test;
+-- A BEFORE UPDATE trigger that changes the application-time column is allowed,
+-- even if the results are senseless.
+-- Note this is likely to cause a primary key violation.
+CREATE TABLE for_portion_of_test (
+  id int4range,
+  valid_at daterange,
+  name text
+);
+CREATE FUNCTION trg_fpo_change_valid_at()
+RETURNS TRIGGER LANGUAGE plpgsql AS
+$$
+BEGIN
+  NEW.valid_at = daterange('2018-01-01', '2019-01-01');
+  RETURN NEW;
+END;
+$$;
+CREATE TRIGGER fpo_before_update_row
+  BEFORE UPDATE ON for_portion_of_test
+  FOR EACH ROW EXECUTE PROCEDURE trg_fpo_change_valid_at();
+INSERT INTO for_portion_of_test VALUES ('[1,2)', '[2010-01-01,2020-01-01)', 'foo');
+UPDATE for_portion_of_test
+  FOR PORTION OF valid_at FROM '2018-05-01' TO '2018-06-01'
+  SET name = CONCAT(name, '!')
+  WHERE id = '[1,2)';
+SELECT * FROM for_portion_of_test ORDER BY id, valid_at;
+  id   |        valid_at         | name 
+-------+-------------------------+------
+ [1,2) | [2010-01-01,2018-05-01) | foo
+ [1,2) | [2018-01-01,2019-01-01) | foo!
+ [1,2) | [2018-06-01,2020-01-01) | foo
+(3 rows)
+
+-- A primary key should reject anything invalid:
+TRUNCATE for_portion_of_test;
+ALTER TABLE for_portion_of_test
+  ADD CONSTRAINT for_portion_of_test_key
+  PRIMARY KEY (id, valid_at WITHOUT OVERLAPS);
+INSERT INTO for_portion_of_test VALUES ('[1,2)', '[2010-01-01,2020-01-01)', 'foo');
+UPDATE for_portion_of_test
+  FOR PORTION OF valid_at FROM '2018-05-01' TO '2018-06-01'
+  SET name = CONCAT(name, '!')
+  WHERE id = '[1,2)';
+ERROR:  conflicting key value violates exclusion constraint "for_portion_of_test_key"
+DETAIL:  Key (id, valid_at)=([1,2), [2010-01-01,2018-05-01)) conflicts with existing key (id, valid_at)=([1,2), [2018-01-01,2019-01-01)).
+DROP TRIGGER fpo_before_update_row ON for_portion_of_test;
+DROP FUNCTION trg_fpo_change_valid_at();
+DROP TABLE for_portion_of_test;
 -- Test with multiranges
 CREATE TABLE for_portion_of_test2 (
   id int4range NOT NULL,
diff --git a/src/test/regress/sql/for_portion_of.sql b/src/test/regress/sql/for_portion_of.sql
index f48644347d1..a8d29a76b22 100644
--- a/src/test/regress/sql/for_portion_of.sql
+++ b/src/test/regress/sql/for_portion_of.sql
@@ -1215,6 +1215,53 @@ SELECT * FROM for_portion_of_test ORDER BY valid_at;
 DROP FUNCTION fpo_append_name_suffix CASCADE;
 DROP TABLE for_portion_of_test;
 
+-- A BEFORE UPDATE trigger that changes the application-time column is allowed,
+-- even if the results are senseless.
+-- Note this is likely to cause a primary key violation.
+
+CREATE TABLE for_portion_of_test (
+  id int4range,
+  valid_at daterange,
+  name text
+);
+
+CREATE FUNCTION trg_fpo_change_valid_at()
+RETURNS TRIGGER LANGUAGE plpgsql AS
+$$
+BEGIN
+  NEW.valid_at = daterange('2018-01-01', '2019-01-01');
+  RETURN NEW;
+END;
+$$;
+
+CREATE TRIGGER fpo_before_update_row
+  BEFORE UPDATE ON for_portion_of_test
+  FOR EACH ROW EXECUTE PROCEDURE trg_fpo_change_valid_at();
+
+INSERT INTO for_portion_of_test VALUES ('[1,2)', '[2010-01-01,2020-01-01)', 'foo');
+
+UPDATE for_portion_of_test
+  FOR PORTION OF valid_at FROM '2018-05-01' TO '2018-06-01'
+  SET name = CONCAT(name, '!')
+  WHERE id = '[1,2)';
+
+SELECT * FROM for_portion_of_test ORDER BY id, valid_at;
+
+-- A primary key should reject anything invalid:
+TRUNCATE for_portion_of_test;
+ALTER TABLE for_portion_of_test
+  ADD CONSTRAINT for_portion_of_test_key
+  PRIMARY KEY (id, valid_at WITHOUT OVERLAPS);
+INSERT INTO for_portion_of_test VALUES ('[1,2)', '[2010-01-01,2020-01-01)', 'foo');
+UPDATE for_portion_of_test
+  FOR PORTION OF valid_at FROM '2018-05-01' TO '2018-06-01'
+  SET name = CONCAT(name, '!')
+  WHERE id = '[1,2)';
+
+DROP TRIGGER fpo_before_update_row ON for_portion_of_test;
+DROP FUNCTION trg_fpo_change_valid_at();
+DROP TABLE for_portion_of_test;
+
 -- Test with multiranges
 
 CREATE TABLE for_portion_of_test2 (