diff --git a/scripts/yau07.gd b/scripts/yau07.gd
index 75665a27..8c7619e7 100644
--- a/scripts/yau07.gd
+++ b/scripts/yau07.gd
@@ -236,10 +236,10 @@ class YaU07 extends unit.Unit:
         var block_len = rx_data.decode_u16(ports_count_pos + 3)
         var block_adr =  rx_data.decode_u16(ports_count_pos + 5)
         var block_data = []
+        if rx_data.size() < (ports_count_pos + 7 + (block_len - 1) * 2):
+                return false
         for i in block_len:
             var shift: int = i * 2
-            if rx_data.size() < (ports_count_pos + 7 + shift):
-                return false
             block_data.append(rx_data.decode_u16(ports_count_pos + 7 + shift))
         isa_block[block_adr] = block_data
         return true