diff --git a/src/bin/scripts/vacuumdb.c b/src/bin/scripts/vacuumdb.c
index 1034a1f3c4..5131bd9198 100644
--- a/src/bin/scripts/vacuumdb.c
+++ b/src/bin/scripts/vacuumdb.c
@@ -200,12 +200,6 @@ main(int argc, char *argv[])
 							progname);
 					exit(1);
 				}
-				if (concurrentCons > FD_SETSIZE - 1)
-				{
-					fprintf(stderr, _("%s: too many parallel jobs requested (maximum: %d)\n"),
-							progname, FD_SETSIZE - 1);
-					exit(1);
-				}
 				break;
 			case 2:
 				maintenance_db = pg_strdup(optarg);
@@ -442,6 +436,20 @@ vacuum_one_database(const char *dbname, vacuumingOptions *vacopts,
 		{
 			conn = connectDatabase(dbname, host, port, username, prompt_password,
 								   progname, echo, false, true);
+
+			/*
+			 * Fail and exit immediately if trying to use a socket in an
+			 * unsupported range.  POSIX requires open(2) to use the lowest
+			 * unused file descriptor and the hint given relies on that.
+			 */
+			if (PQsocket(conn) >= FD_SETSIZE)
+			{
+				fprintf(stderr,
+						_("%s: too many jobs for this platform -- try %d"),
+						progname, i);
+				exit(1);
+			}
+
 			init_slot(slots + i, conn);
 		}
 	}