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
@@ -96,13 +96,16 @@
         self.nodes[0].setexcessiveblock(self.excessive_block_size)
         self.test.run()
 
+    def add_transactions_to_block(self, block, tx_list):
+        [tx.rehash() for tx in tx_list]
+        block.vtx.extend(tx_list)
+
     # this is a little handier to use than the version in blocktools.py
     def create_tx(self, spend_tx, n, value, script=CScript([OP_TRUE])):
         tx = create_transaction(spend_tx, n, b"", value, script)
         return tx
 
     def next_block(self, number, spend=None, script=CScript([OP_TRUE]), block_size=0, extra_txns=0):
-        assert number not in self.blocks
         if self.tip == None:
             base_block_hash = self.genesis_hash
             block_time = int(time.time()) + 1
@@ -154,11 +157,11 @@
                 CTxOut(0, CScript([random.randint(0, 256), OP_RETURN])))
 
             # Add the transaction to the block
-            block.vtx.append(tx)
+            self.add_transactions_to_block(block, [tx])
 
             # Add transaction until we reach the expected transaction count
-            block.vtx.extend([get_base_transaction()
-                              for _ in range(extra_txns)])
+            for _ in range(extra_txns):
+                self.add_transactions_to_block(block, [get_base_transaction()])
 
             # If we have a block size requirement, just fill
             # the block until we get there
@@ -187,7 +190,7 @@
 
                 # Add the tx to the list of transactions to be included
                 # in the block.
-                block.vtx.append(tx)
+                self.add_transactions_to_block(block, [tx])
                 current_block_size += len(tx.serialize())
 
             # Now that we added a bunch of transaction, we need to recompute
@@ -203,6 +206,7 @@
         block.solve()
         self.tip = block
         self.block_heights[block.sha256] = height
+        assert number not in self.blocks
         self.blocks[number] = block
         return block