diff --git a/src/backend/executor/execTuples.c b/src/backend/executor/execTuples.c
index 97ae019d10a..3ad983c7fa5 100644
--- a/src/backend/executor/execTuples.c
+++ b/src/backend/executor/execTuples.c
@@ -1254,7 +1254,17 @@ slot_deform_heap_tuple(TupleTableSlot *slot, HeapTuple tuple, uint32 *offp,
 		 * to implement a tail-call optimization
 		 */
 		*offp = off;
-		slot_getmissingattrs(slot, attnum, reqnatts);
+
+		Assert(HeapTupleHeaderGetNatts(tup) <= attnum);
+
+		/*
+		 * Fetch all missing attributes.  We pass natts rather than attnum as
+		 * if we're deforming attributes after having already deformed some
+		 * missing attributes, then the call to populate_isnull_array() may
+		 * have overwritten the previous tts_isnull values from what was
+		 * stored in the previous call to slot_getmissingattrs().
+		 */
+		slot_getmissingattrs(slot, HeapTupleHeaderGetNatts(tup), reqnatts);
 		return;
 	}
 done:
diff --git a/src/test/regress/expected/fast_default.out b/src/test/regress/expected/fast_default.out
index 49485f5ec6a..20356467655 100644
--- a/src/test/regress/expected/fast_default.out
+++ b/src/test/regress/expected/fast_default.out
@@ -944,6 +944,21 @@ SELECT * FROM t ORDER BY id;
 (3 rows)
 
 DROP TABLE t;
+-- Ensure defaults are correctly applied during tuple deformation for whole
+-- row vars when the defaults are deformed incrementally.
+CREATE TABLE t_missing_wholerow (a int NOT NULL, b int);
+INSERT INTO t_missing_wholerow (a, b) VALUES (1, null); -- with NULL bitmap
+INSERT INTO t_missing_wholerow (a, b) VALUES (2, 3); -- without NULL bitmap
+ALTER TABLE t_missing_wholerow ADD COLUMN c int NOT NULL DEFAULT 15;
+ALTER TABLE t_missing_wholerow ADD COLUMN d bigint NOT NULL DEFAULT 25;
+SELECT a, b, c, t FROM t_missing_wholerow t;
+ a | b | c  |      t      
+---+---+----+-------------
+ 1 |   | 15 | (1,,15,25)
+ 2 | 3 | 15 | (2,3,15,25)
+(2 rows)
+
+DROP TABLE t_missing_wholerow;
 -- cleanup
 DROP FOREIGN TABLE ft1;
 DROP SERVER s0;
diff --git a/src/test/regress/sql/fast_default.sql b/src/test/regress/sql/fast_default.sql
index ccd138c4efc..36f81a184f0 100644
--- a/src/test/regress/sql/fast_default.sql
+++ b/src/test/regress/sql/fast_default.sql
@@ -618,6 +618,16 @@ REPACK t;
 SELECT * FROM t ORDER BY id;
 DROP TABLE t;
 
+-- Ensure defaults are correctly applied during tuple deformation for whole
+-- row vars when the defaults are deformed incrementally.
+CREATE TABLE t_missing_wholerow (a int NOT NULL, b int);
+INSERT INTO t_missing_wholerow (a, b) VALUES (1, null); -- with NULL bitmap
+INSERT INTO t_missing_wholerow (a, b) VALUES (2, 3); -- without NULL bitmap
+ALTER TABLE t_missing_wholerow ADD COLUMN c int NOT NULL DEFAULT 15;
+ALTER TABLE t_missing_wholerow ADD COLUMN d bigint NOT NULL DEFAULT 25;
+SELECT a, b, c, t FROM t_missing_wholerow t;
+DROP TABLE t_missing_wholerow;
+
 -- cleanup
 DROP FOREIGN TABLE ft1;
 DROP SERVER s0;