diff --git a/src/backend/commands/subscriptioncmds.c b/src/backend/commands/subscriptioncmds.c
index ee0983fd87..fde9e6e20c 100644
--- a/src/backend/commands/subscriptioncmds.c
+++ b/src/backend/commands/subscriptioncmds.c
@@ -653,6 +653,11 @@ AlterSubscription(AlterSubscriptionStmt *stmt)
 			}
 
 		case ALTER_SUBSCRIPTION_CONNECTION:
+			/* Load the library providing us libpq calls. */
+			load_file("libpqwalreceiver", false);
+			/* Check the connection info string. */
+			walrcv_check_conninfo(stmt->conninfo);
+
 			values[Anum_pg_subscription_subconninfo - 1] =
 				CStringGetTextDatum(stmt->conninfo);
 			replaces[Anum_pg_subscription_subconninfo - 1] = true;
diff --git a/src/test/regress/expected/subscription.out b/src/test/regress/expected/subscription.out
index b1686db12e..fd09f54548 100644
--- a/src/test/regress/expected/subscription.out
+++ b/src/test/regress/expected/subscription.out
@@ -45,6 +45,10 @@ SET SESSION AUTHORIZATION 'regress_subscription_user2';
 CREATE SUBSCRIPTION testsub2 CONNECTION 'dbname=doesnotexist' PUBLICATION foo WITH (NOCONNECT);
 ERROR:  must be superuser to create subscriptions
 SET SESSION AUTHORIZATION 'regress_subscription_user';
+-- fail - invalid connection string
+ALTER SUBSCRIPTION testsub CONNECTION 'foobar';
+ERROR:  invalid connection string syntax: missing "=" after "foobar" in connection info string
+
 \dRs+
                                          List of subscriptions
   Name   |           Owner           | Enabled | Publication | Synchronous commit |      Conninfo       
diff --git a/src/test/regress/sql/subscription.sql b/src/test/regress/sql/subscription.sql
index 1b30d150ce..db05f523a2 100644
--- a/src/test/regress/sql/subscription.sql
+++ b/src/test/regress/sql/subscription.sql
@@ -38,6 +38,9 @@ SET SESSION AUTHORIZATION 'regress_subscription_user2';
 CREATE SUBSCRIPTION testsub2 CONNECTION 'dbname=doesnotexist' PUBLICATION foo WITH (NOCONNECT);
 SET SESSION AUTHORIZATION 'regress_subscription_user';
 
+-- fail - invalid connection string
+ALTER SUBSCRIPTION testsub CONNECTION 'foobar';
+
 \dRs+
 
 ALTER SUBSCRIPTION testsub SET PUBLICATION testpub2, testpub3 NOREFRESH;