diff --git a/test/functional/abc-p2p-compactblocks.py b/test/functional/abc-p2p-compactblocks.py
--- a/test/functional/abc-p2p-compactblocks.py
+++ b/test/functional/abc-p2p-compactblocks.py
@@ -175,9 +175,12 @@
 
                 # Create the new transaction
                 tx = get_base_transaction()
+                pad_tx(tx)
+                tx.rehash()
+                tx_size = len(tx.serialize()) + 18
 
                 # Add padding to fill the block.
-                script_length = block_size - current_block_size - base_tx_size
+                script_length = block_size - current_block_size - tx_size
                 if script_length > 510000:
                     if script_length < 1000000:
                         # Make sure we don't find ourselves in a position where we