diff --git a/internal/platform/prng_test.cc b/internal/platform/prng_test.cc
index e6287aa4..68538fbb 100644
--- a/internal/platform/prng_test.cc
+++ b/internal/platform/prng_test.cc
@@ -50,9 +50,11 @@ TEST(PrngTest, NextInt32GeneratesDifferentValues) {
 }
 
 TEST(PrngTest, NextInt32AcrossThreads) {
+  absl::Mutex mutex;
   Prng prng;
   std::set<int32_t> values;
-  auto func = [&prng, &values]() {
+  auto func = [&prng, &values, &mutex]() {
+    absl::MutexLock lock(&mutex);
     auto value = prng.NextInt32();
     EXPECT_LE(value, std::numeric_limits<std::int32_t>::max());
     EXPECT_GE(value, std::numeric_limits<std::int32_t>::min());