diff --git a/test_runner/performance/test_wal_backpressure.py b/test_runner/performance/test_wal_backpressure.py
index cb35cad46b..dd840acd25 100644
--- a/test_runner/performance/test_wal_backpressure.py
+++ b/test_runner/performance/test_wal_backpressure.py
@@ -216,7 +216,7 @@ def record_lsn_write_lag(env: PgCompare, run_cond: Callable[[], bool], pool_inte
             )
 
             res = cur.fetchone()
-            assert isinstance(res, list)
+            assert isinstance(res, tuple)
             lsn_write_lags.append(res[0])
 
             curr_received_lsn = Lsn(res[3])