diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 8ba1c170f0..fd471d74a3 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -4115,8 +4115,15 @@ PostgresMain(const char *dbname, const char *username)
     /*
      * process any libraries that should be preloaded at backend start (this
      * likewise can't be done until GUC settings are complete)
+     *
+     * If the user provided a setting at session startup for a custom GUC
+     * defined by one of these libraries, we might need syscache access when
+     * evaluating whether she has permission to set it, so do this step within
+     * a transaction.
      */
+    StartTransactionCommand();
     process_session_preload_libraries();
+    CommitTransactionCommand();
 
     /*
      * Send this backend's cancellation info to the frontend.