*** a/src/backend/replication/syncrep.c
--- b/src/backend/replication/syncrep.c
***************
*** 924,929 **** check_synchronous_standby_names(char **newval, void **extra, GucSource source)
--- 924,936 ----
  			return false;
  		}
  
+ 		if (syncrep_parse_result->num_sync <= 0)
+ 		{
+ 			GUC_check_errmsg("number of synchronous standbys (%d) must be greater than zero",
+ 							 syncrep_parse_result->num_sync);
+ 			return false;
+ 		}
+ 
  		/* GUC extra value must be malloc'd, not palloc'd */
  		pconf = (SyncRepConfigData *)
  			malloc(syncrep_parse_result->config_size);