diff -Naur rsync-3.1.3/options.c rsync-3.1.3-2/options.c
--- rsync-3.1.3/options.c	2018-01-15 13:55:07.000000000 +1000
+++ rsync-3.1.3-2/options.c	2019-05-29 12:59:26.470327515 +1000
@@ -1977,9 +1977,9 @@
 	}
 #endif
 
-	if (block_size > MAX_BLOCK_SIZE) {
+	if ( (protocol_version >= 30 & block_size > MAX_BLOCK_SIZE) || (protocol_version < 30 & block_size > OLD_MAX_BLOCK_SIZE) ) {
 		snprintf(err_buf, sizeof err_buf,
-			 "--block-size=%lu is too large (max: %u)\n", block_size, MAX_BLOCK_SIZE);
+			 "--block-size=%lu is too large (max: %u)\n", block_size, protocol_version < 30 ? OLD_MAX_BLOCK_SIZE : MAX_BLOCK_SIZE);
 		return 0;
 	}