diff --git a/innobase/include/rem0rec.h b/innobase/include/rem0rec.h
index 69b397c9682..b573c5d4c3b 100644
--- a/innobase/include/rem0rec.h
+++ b/innobase/include/rem0rec.h
@@ -368,8 +368,9 @@ rec_set_field_extern_bits(
 /*************************************************************** 
 This is used to modify the value of an already existing field in a record.
 The previous value must have exactly the same size as the new value. If len
-is UNIV_SQL_NULL then the field is treated as an SQL null for old-style
-records. For new-style records, len must not be UNIV_SQL_NULL. */
+is UNIV_SQL_NULL then the field is treated as an SQL null.
+For records in ROW_FORMAT=COMPACT (new-style records), len must not be
+UNIV_SQL_NULL unless the field already is SQL null. */
 UNIV_INLINE
 void
 rec_set_nth_field(
@@ -378,11 +379,7 @@ rec_set_nth_field(
 	const ulint*	offsets,/* in: array returned by rec_get_offsets() */
 	ulint		n,	/* in: index number of the field */
 	const void*	data,	/* in: pointer to the data if not SQL null */
-	ulint		len);	/* in: length of the data or UNIV_SQL_NULL.
-				If not SQL null, must have the same
-				length as the previous value.
-				If SQL null, previous value must be
-				SQL null. */
+	ulint		len);	/* in: length of the data or UNIV_SQL_NULL */
 /************************************************************** 
 The following function returns the data size of an old-style physical
 record, that is the sum of field lengths. SQL null fields
diff --git a/innobase/include/rem0rec.ic b/innobase/include/rem0rec.ic
index 1abbb503bab..64c91724386 100644
--- a/innobase/include/rem0rec.ic
+++ b/innobase/include/rem0rec.ic
@@ -1204,8 +1204,9 @@ rec_get_nth_field_size(
 /*************************************************************** 
 This is used to modify the value of an already existing field in a record.
 The previous value must have exactly the same size as the new value. If len
-is UNIV_SQL_NULL then the field is treated as an SQL null for old-style
-records. For new-style records, len must not be UNIV_SQL_NULL. */
+is UNIV_SQL_NULL then the field is treated as an SQL null.
+For records in ROW_FORMAT=COMPACT (new-style records), len must not be
+UNIV_SQL_NULL unless the field already is SQL null. */
 UNIV_INLINE
 void
 rec_set_nth_field(
@@ -1215,11 +1216,7 @@ rec_set_nth_field(
 	ulint		n,	/* in: index number of the field */
 	const void*	data,	/* in: pointer to the data
 				if not SQL null */
-	ulint		len)	/* in: length of the data or UNIV_SQL_NULL.
-				If not SQL null, must have the same
-				length as the previous value.
-				If SQL null, previous value must be
-				SQL null. */
+	ulint		len)	/* in: length of the data or UNIV_SQL_NULL */
 {
 	byte*	data2;
 	ulint	len2;
@@ -1227,9 +1224,11 @@ rec_set_nth_field(
 	ut_ad(rec);
 	ut_ad(rec_offs_validate(rec, NULL, offsets));
 
-	if (len == UNIV_SQL_NULL) {
-		ut_ad(!rec_offs_comp(offsets));
-		rec_set_nth_field_sql_null(rec, n);
+	if (UNIV_UNLIKELY(len == UNIV_SQL_NULL)) {
+		if (!rec_offs_nth_sql_null(offsets, n)) {
+			ut_a(!rec_offs_comp(offsets));
+			rec_set_nth_field_sql_null(rec, n);
+		}
 
 		return;
 	}