diff --git a/src/backend/access/common/tupconvert.c b/src/backend/access/common/tupconvert.c
index 2d0d2f4b32..a40382b146 100644
--- a/src/backend/access/common/tupconvert.c
+++ b/src/backend/access/common/tupconvert.c
@@ -149,8 +149,18 @@ convert_tuples_by_position(TupleDesc indesc,
 	{
 		for (i = 0; i < n; 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)
+			{
+				same = false;
+				break;
+			}
 
 			if (attrMap[i] == (i + 1))
 				continue;
@@ -160,8 +170,6 @@ convert_tuples_by_position(TupleDesc indesc,
 			 * also dropped, we needn't convert.  However, attlen and attalign
 			 * must agree.
 			 */
-			inatt = TupleDescAttr(indesc, i);
-			outatt = TupleDescAttr(outdesc, i);
 			if (attrMap[i] == 0 &&
 				inatt->attisdropped &&
 				inatt->attlen == outatt->attlen &&
@@ -233,8 +241,18 @@ convert_tuples_by_name(TupleDesc indesc,
 		same = true;
 		for (i = 0; i < n; 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)
+			{
+				same = false;
+				break;
+			}
 
 			if (attrMap[i] == (i + 1))
 				continue;
@@ -244,8 +262,6 @@ convert_tuples_by_name(TupleDesc indesc,
 			 * also dropped, we needn't convert.  However, attlen and attalign
 			 * must agree.
 			 */
-			inatt = TupleDescAttr(indesc, i);
-			outatt = TupleDescAttr(outdesc, i);
 			if (attrMap[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 44c178ff4a..4e9c15bbd4 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -4174,3 +4174,41 @@ 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 c65791d4d3..cfc09e859b 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -2763,3 +2763,42 @@ 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 */