diff --git a/test/fixtures/guess-hash-seed.js b/test/fixtures/guess-hash-seed.js
index 8d69cf92490d20..c6166450b4fbb5 100644
--- a/test/fixtures/guess-hash-seed.js
+++ b/test/fixtures/guess-hash-seed.js
@@ -67,9 +67,6 @@ function hash_to_bucket(hash, numBuckets) {
 function time_set_lookup(set, value) {
   const t1 = process.hrtime();
   for (let i = 0; i < 100; i++) {
-    // annoyingly, SetHas() is JS code and therefore potentially optimizable.
-    // However, SetHas() looks up the table using native code, and it seems like
-    // that's sufficient to prevent the optimizer from doing anything?
     set.has(value);
   }
   const t = process.hrtime(t1);
@@ -78,6 +75,9 @@ function time_set_lookup(set, value) {
   return secs * 1e9 + nanos;
 }
 
+// Prevent optimization of SetHas().
+%NeverOptimizeFunction(time_set_lookup);
+
 // Set with 256 buckets; bucket 0 full, others empty
 const tester_set_buckets = 256;
 const tester_set = new Set();
diff --git a/test/pummel/test-hash-seed.js b/test/pummel/test-hash-seed.js
index 274183d8ce977e..265226e1714fe2 100644
--- a/test/pummel/test-hash-seed.js
+++ b/test/pummel/test-hash-seed.js
@@ -24,7 +24,14 @@ const requiredCallback = common.mustCall((results) => {
   assert.strictEqual(seeds.length, kRepetitions);
 });
 
-const generateSeed = () => execFilePromise(process.execPath, [targetScript]);
+function generateSeed() {
+  return execFilePromise(process.execPath, [
+    // Needed for %NeverOptimizeFunction.
+    '--allow-natives-syntax',
+    targetScript,
+  ]);
+}
+
 const subprocesses = [...new Array(kRepetitions)].map(generateSeed);
 
 Promise.all(subprocesses)