diff --git a/src/backend/catalog/heap.c b/src/backend/catalog/heap.c
index e997b57..f6f0691 100644
--- a/src/backend/catalog/heap.c
+++ b/src/backend/catalog/heap.c
@@ -105,7 +105,7 @@ static void StoreConstraints(Relation rel, List *cooked_constraints,
 				 bool is_internal);
 static bool MergeWithExistingConstraint(Relation rel, char *ccname, Node *expr,
 							bool allow_merge, bool is_local,
-							bool is_no_inherit);
+							bool is_no_inherit, bool is_not_valid);
 static void SetRelationNumChecks(Relation rel, int numchecks);
 static Node *cookConstraint(ParseState *pstate,
 			   Node *raw_constraint,
@@ -2301,7 +2301,8 @@ AddRelationNewConstraints(Relation rel,
 			 */
 			if (MergeWithExistingConstraint(rel, ccname, expr,
 											allow_merge, is_local,
-											cdef->is_no_inherit))
+											cdef->is_no_inherit,
+											cdef->skip_validation))
 				continue;
 		}
 		else
@@ -2389,7 +2390,7 @@ AddRelationNewConstraints(Relation rel,
 static bool
 MergeWithExistingConstraint(Relation rel, char *ccname, Node *expr,
 							bool allow_merge, bool is_local,
-							bool is_no_inherit)
+							bool is_no_inherit, bool is_not_valid)
 {
 	bool		found;
 	Relation	conDesc;
@@ -2452,6 +2453,14 @@ MergeWithExistingConstraint(Relation rel, char *ccname, Node *expr,
 						 errmsg("constraint \"%s\" conflicts with non-inherited constraint on relation \"%s\"",
 								ccname, RelationGetRelationName(rel))));
 
+			/* Cannot merge if the child's constraint is yet invalid. */
+			if (!is_local && !is_not_valid && !con->convalidated)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						 errmsg("cannot merge constraint \"%s\" with NOT VALID constraint on relation \"%s\"",
+								ccname, RelationGetRelationName(rel)),
+						 errhint("Please validate the constraint on the child table first.")));
+
 			if (is_local)
 				con->conislocal = true;
 			else
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 86e9814..5e72029 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -10380,6 +10380,14 @@ MergeConstraintsIntoExisting(Relation child_rel, Relation parent_rel)
 								NameStr(child_con->conname),
 								RelationGetRelationName(child_rel))));
 
+			/* Cannot merge if the child's constraint is yet invalid. */
+			if (parent_con->convalidated && !child_con->convalidated)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						 errmsg("cannot merge constraint \"%s\" with NOT VALID constraint on child table \"%s\"",
+								 NameStr(parent_con->conname), RelationGetRelationName(child_rel)),
+						 errhint("Please validate the constraint on the child table first.")));
+
 			/*
 			 * OK, bump the child constraint's inheritance count.  (If we fail
 			 * later on, this change will just roll back.)
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index 3232cda..9f13c47 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -460,6 +460,43 @@ Check constraints:
 Inherits: nv_parent
 
 -- we leave nv_parent and children around to help test pg_dump logic
+-- prevent merge of constraint with NOT VALID constraint on a child table
+create table parent(a int);
+create table child () inherits (parent);
+alter table child add constraint chk_a check (a > 0) not valid;
+-- ok to merge a NOT VALID constraint itself
+alter table parent add constraint chk_a check (a > 0) not valid;
+NOTICE:  merging constraint "chk_a" with inherited definition
+alter table parent drop constraint chk_a;
+-- error
+alter table parent add constraint chk_a check (a > 0);
+ERROR:  cannot merge constraint "chk_a" with NOT VALID constraint on relation "child"
+HINT:  Please validate the constraint on the child table first.
+-- ok to merge upon validate
+alter table child validate constraint chk_a;
+alter table parent add constraint chk_a check (a > 0);
+NOTICE:  merging constraint "chk_a" with inherited definition
+-- check the ALTER TABLE INHERIT case as well
+drop table child;
+create table child(a int);
+alter table child add constraint chk_a check (a > 0) not valid;
+-- ok to merge a NOT VALID constraint itself
+alter table parent drop constraint chk_a;
+alter table parent add constraint chk_a check (a > 0) not valid;
+alter table child inherit parent;
+alter table child no inherit parent;
+alter table parent drop constraint chk_a;
+alter table parent add constraint chk_a check (a > 0);
+-- error
+alter table child inherit parent;
+ERROR:  cannot merge constraint "chk_a" with NOT VALID constraint on child table "child"
+HINT:  Please validate the constraint on the child table first.
+-- ok to merge upon validate
+alter table child validate constraint chk_a;
+alter table child inherit parent;
+-- clean up
+drop table parent cascade;
+NOTICE:  drop cascades to table child
 -- Foreign key adding test with mixed types
 -- Note: these tables are TEMP to avoid name conflicts when this test
 -- is run in parallel with foreign_key.sql.
diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index 72e65d4..55bf2e9 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -348,6 +348,45 @@ alter table nv_parent add check (d between '2001-01-01'::date and '2099-12-31'::
 \d nv_child_2009
 -- we leave nv_parent and children around to help test pg_dump logic
 
+-- prevent merge of constraint with NOT VALID constraint on a child table
+
+create table parent(a int);
+create table child () inherits (parent);
+alter table child add constraint chk_a check (a > 0) not valid;
+
+-- ok to merge a NOT VALID constraint itself
+alter table parent add constraint chk_a check (a > 0) not valid;
+
+alter table parent drop constraint chk_a;
+-- error
+alter table parent add constraint chk_a check (a > 0);
+-- ok to merge upon validate
+alter table child validate constraint chk_a;
+alter table parent add constraint chk_a check (a > 0);
+
+-- check the ALTER TABLE INHERIT case as well
+
+drop table child;
+create table child(a int);
+alter table child add constraint chk_a check (a > 0) not valid;
+
+-- ok to merge a NOT VALID constraint itself
+alter table parent drop constraint chk_a;
+alter table parent add constraint chk_a check (a > 0) not valid;
+alter table child inherit parent;
+
+alter table child no inherit parent;
+alter table parent drop constraint chk_a;
+alter table parent add constraint chk_a check (a > 0);
+-- error
+alter table child inherit parent;
+-- ok to merge upon validate
+alter table child validate constraint chk_a;
+alter table child inherit parent;
+
+-- clean up
+drop table parent cascade;
+
 -- Foreign key adding test with mixed types
 
 -- Note: these tables are TEMP to avoid name conflicts when this test