diff --git a/src/backend/access/common/attmap.c b/src/backend/access/common/attmap.c
index a074852a015..2cd16d7eafb 100644
--- a/src/backend/access/common/attmap.c
+++ b/src/backend/access/common/attmap.c
@@ -294,8 +294,14 @@ check_attrmap_match(TupleDesc indesc,
 
 	for (i = 0; i < attrMap->maplen; i++)
 	{
-		Form_pg_attribute inatt;
-		Form_pg_attribute outatt;
+		Form_pg_attribute inatt = TupleDescAttr(indesc, i);
+		Form_pg_attribute outatt = TupleDescAttr(outdesc, i);
+
+		/*
+		 * If the input column has a missing attribute, we need a conversion.
+		 */
+		if (inatt->atthasmissing)
+			return false;
 
 		if (attrMap->attnums[i] == (i + 1))
 			continue;
@@ -305,8 +311,6 @@ check_attrmap_match(TupleDesc indesc,
 		 * dropped, we don't need a conversion.  However, attlen and attalign
 		 * must agree.
 		 */
-		inatt = TupleDescAttr(indesc, i);
-		outatt = TupleDescAttr(outdesc, i);
 		if (attrMap->attnums[i] == 0 &&
 			inatt->attisdropped &&
 			inatt->attlen == outatt->attlen &&
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index e73ce4b6f3c..fb6d86a2690 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -4258,3 +4258,41 @@ alter table at_test_sql_partop attach partition at_test_sql_partop_1 for values
 drop table at_test_sql_partop;
 drop operator class at_test_sql_partop using btree;
 drop function at_test_sql_partop;
+/* Test case for bug #16242 */
+-- We create a parent and child where the child has missing
+-- non-null attribute values, and arrange to pass them through
+-- tuple conversion from the child to the parent tupdesc
+create table bar1 (a integer, b integer not null default 1)
+  partition by range (a);
+create table bar2 (a integer);
+insert into bar2 values (1);
+alter table bar2 add column b integer not null default 1;
+-- (at this point bar2 contains tuple with natts=1)
+alter table bar1 attach partition bar2 default;
+-- this works:
+select * from bar1;
+ a | b 
+---+---
+ 1 | 1
+(1 row)
+
+-- this exercises tuple conversion:
+create function xtrig()
+  returns trigger language plpgsql
+as $$
+  declare
+    r record;
+  begin
+    for r in select * from old loop
+      raise info 'a=%, b=%', r.a, r.b;
+    end loop;
+    return NULL;
+  end;
+$$;
+create trigger xtrig
+  after update on bar1
+  referencing old table as old
+  for each statement execute procedure xtrig();
+update bar1 set a = a + 1;
+INFO:  a=1, b=1
+/* End test case for bug #16242 */
diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index a16e4c9a293..3801f19c58e 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -2801,3 +2801,42 @@ alter table at_test_sql_partop attach partition at_test_sql_partop_1 for values
 drop table at_test_sql_partop;
 drop operator class at_test_sql_partop using btree;
 drop function at_test_sql_partop;
+
+
+/* Test case for bug #16242 */
+
+-- We create a parent and child where the child has missing
+-- non-null attribute values, and arrange to pass them through
+-- tuple conversion from the child to the parent tupdesc
+create table bar1 (a integer, b integer not null default 1)
+  partition by range (a);
+create table bar2 (a integer);
+insert into bar2 values (1);
+alter table bar2 add column b integer not null default 1;
+-- (at this point bar2 contains tuple with natts=1)
+alter table bar1 attach partition bar2 default;
+
+-- this works:
+select * from bar1;
+
+-- this exercises tuple conversion:
+create function xtrig()
+  returns trigger language plpgsql
+as $$
+  declare
+    r record;
+  begin
+    for r in select * from old loop
+      raise info 'a=%, b=%', r.a, r.b;
+    end loop;
+    return NULL;
+  end;
+$$;
+create trigger xtrig
+  after update on bar1
+  referencing old table as old
+  for each statement execute procedure xtrig();
+
+update bar1 set a = a + 1;
+
+/* End test case for bug #16242 */