diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index a4ca363f20d..1e9f4602c69 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -2784,53 +2784,32 @@ GetSlotInvalidationCauseName(ReplicationSlotInvalidationCause cause)
 static bool
 validate_sync_standby_slots(char *rawname, List **elemlist)
 {
-	bool		ok;
-
 	/* Verify syntax and parse string into a list of identifiers */
-	ok = SplitIdentifierString(rawname, ',', elemlist);
-
-	if (!ok)
+	if (!SplitIdentifierString(rawname, ',', elemlist))
 	{
 		GUC_check_errdetail("List syntax is invalid.");
+		return false;
 	}
-	else if (MyProc)
+
+	/* Iterate the list to validate each slot name */
+	foreach_ptr(char, name, *elemlist)
 	{
-		/*
-		 * Check that each specified slot exist and is physical.
-		 *
-		 * Because we need an LWLock, we cannot do this on processes without a
-		 * PGPROC, so we skip it there; but see comments in
-		 * StandbySlotsHaveCaughtup() as to why that's not a problem.
-		 */
-		LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
+		int			err_code;
+		char	   *err_msg = NULL;
+		char	   *err_hint = NULL;
 
-		foreach_ptr(char, name, *elemlist)
+		if (!ReplicationSlotValidateNameInternal(name, false, &err_code,
+												 &err_msg, &err_hint))
 		{
-			ReplicationSlot *slot;
-
-			slot = SearchNamedReplicationSlot(name, false);
-
-			if (!slot)
-			{
-				GUC_check_errdetail("Replication slot \"%s\" does not exist.",
-									name);
-				ok = false;
-				break;
-			}
-
-			if (!SlotIsPhysical(slot))
-			{
-				GUC_check_errdetail("\"%s\" is not a physical replication slot.",
-									name);
-				ok = false;
-				break;
-			}
+			GUC_check_errcode(err_code);
+			GUC_check_errdetail("%s", err_msg);
+			if (err_hint != NULL)
+				GUC_check_errhint("%s", err_hint);
+			return false;
 		}
-
-		LWLockRelease(ReplicationSlotControlLock);
 	}
 
-	return ok;
+	return true;
 }
 
 /*
@@ -2988,12 +2967,6 @@ StandbySlotsHaveCaughtup(XLogRecPtr wait_for_lsn, int elevel)
 		/*
 		 * If a slot name provided in synchronized_standby_slots does not
 		 * exist, report a message and exit the loop.
-		 *
-		 * Though validate_sync_standby_slots (the GUC check_hook) tries to
-		 * avoid this, it can nonetheless happen because the user can specify
-		 * a nonexistent slot name before server startup. That function cannot
-		 * validate such a slot during startup, as ReplicationSlotCtl is not
-		 * initialized by then.  Also, the user might have dropped one slot.
 		 */
 		if (!slot)
 		{
diff --git a/src/test/modules/unsafe_tests/expected/guc_privs.out b/src/test/modules/unsafe_tests/expected/guc_privs.out
index 6c0ad898341..3cf2f2fdb85 100644
--- a/src/test/modules/unsafe_tests/expected/guc_privs.out
+++ b/src/test/modules/unsafe_tests/expected/guc_privs.out
@@ -581,6 +581,21 @@ DROP ROLE regress_host_resource_newadmin;  -- ok, nothing was transferred
 -- Use "drop owned by" so we can drop the role
 DROP OWNED BY regress_host_resource_admin;  -- ok
 DROP ROLE regress_host_resource_admin;  -- ok
+-- Test for GUC synchronized standby slots
+-- Cannot set synchronized_standby_slots to a reserved slot name
+ALTER SYSTEM SET synchronized_standby_slots='pg_conflict_detection';
+ERROR:  invalid value for parameter "synchronized_standby_slots": "pg_conflict_detection"
+DETAIL:  replication slot name "pg_conflict_detection" is reserved
+HINT:  The name "pg_conflict_detection" is reserved for the conflict detection slot.
+-- Cannot set synchronized_standby_slots to an invalid slot name
+ALTER SYSTEM SET synchronized_standby_slots='invalid*';
+ERROR:  invalid value for parameter "synchronized_standby_slots": "invalid*"
+DETAIL:  replication slot name "invalid*" contains invalid character
+HINT:  Replication slot names may only contain lower case letters, numbers, and the underscore character.
+-- Can set synchronized_standby_slots to a non-existent slot name
+ALTER SYSTEM SET synchronized_standby_slots='missing';
+-- Reset the GUC
+ALTER SYSTEM RESET synchronized_standby_slots;
 -- Clean up
 RESET SESSION AUTHORIZATION;
 DROP ROLE regress_admin; -- ok
diff --git a/src/test/modules/unsafe_tests/sql/guc_privs.sql b/src/test/modules/unsafe_tests/sql/guc_privs.sql
index 9bcbbfa9040..d0d16f3c29f 100644
--- a/src/test/modules/unsafe_tests/sql/guc_privs.sql
+++ b/src/test/modules/unsafe_tests/sql/guc_privs.sql
@@ -262,6 +262,16 @@ DROP ROLE regress_host_resource_newadmin;  -- ok, nothing was transferred
 DROP OWNED BY regress_host_resource_admin;  -- ok
 DROP ROLE regress_host_resource_admin;  -- ok
 
+-- Test for GUC synchronized standby slots
+-- Cannot set synchronized_standby_slots to a reserved slot name
+ALTER SYSTEM SET synchronized_standby_slots='pg_conflict_detection';
+-- Cannot set synchronized_standby_slots to an invalid slot name
+ALTER SYSTEM SET synchronized_standby_slots='invalid*';
+-- Can set synchronized_standby_slots to a non-existent slot name
+ALTER SYSTEM SET synchronized_standby_slots='missing';
+-- Reset the GUC
+ALTER SYSTEM RESET synchronized_standby_slots;
+
 -- Clean up
 RESET SESSION AUTHORIZATION;
 DROP ROLE regress_admin; -- ok