diff --git a/test/functional/p2p_leak_tx.py b/test/functional/p2p_leak_tx.py
index c0cd86c13..132e3ba42 100755
--- a/test/functional/p2p_leak_tx.py
+++ b/test/functional/p2p_leak_tx.py
@@ -1,63 +1,65 @@
 #!/usr/bin/env python3
 # Copyright (c) 2017-2018 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test that we don't leak txs to inbound peers that we haven't yet announced to"""
 
 from test_framework.messages import MSG_TX, CInv, msg_getdata
-from test_framework.p2p import P2PDataStore
+from test_framework.p2p import P2PDataStore, p2p_lock
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal
 from test_framework.wallet import MiniWallet
 
 
 class P2PNode(P2PDataStore):
     def on_inv(self, msg):
         pass
 
 
 class P2PLeakTxTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 1
 
     def run_test(self):
         # The block and tx generating node
         gen_node = self.nodes[0]
         miniwallet = MiniWallet(gen_node)
         # Add enough mature utxos to the wallet, so that all txs spend
         # confirmed coins
         miniwallet.generate(1)
         gen_node.generate(100)
 
         # An "attacking" inbound peer
         inbound_peer = self.nodes[0].add_p2p_connection(P2PNode())
 
         MAX_REPEATS = 100
         self.log.info("Running test up to {} times.".format(MAX_REPEATS))
         for i in range(MAX_REPEATS):
             self.log.info('Run repeat {}'.format(i + 1))
             txid = miniwallet.send_self_transfer(from_node=gen_node)['txid']
 
             want_tx = msg_getdata()
             want_tx.inv.append(CInv(t=MSG_TX, h=int(txid, 16)))
-            inbound_peer.last_message.pop('notfound', None)
+            with p2p_lock:
+                inbound_peer.last_message.pop('notfound', None)
             inbound_peer.send_and_ping(want_tx)
 
             if inbound_peer.last_message.get('notfound'):
                 self.log.debug(
                     'tx {} was not yet announced to us.'.format(txid))
                 self.log.debug(
                     "node has responded with a notfound message. End test.")
                 assert_equal(
                     inbound_peer.last_message['notfound'].vec[0].hash, int(txid, 16))
-                inbound_peer.last_message.pop('notfound')
+                with p2p_lock:
+                    inbound_peer.last_message.pop('notfound')
                 break
             else:
                 self.log.debug(
                     'tx {} was already announced to us. Try test again.'.format(txid))
                 assert int(txid, 16) in [
                     inv.hash for inv in inbound_peer.last_message['inv'].inv]
 
 
 if __name__ == '__main__':
     P2PLeakTxTest().main()