diff --git a/core/kotlinx-coroutines-core/test/scheduling/CoroutineSchedulerStressTest.kt b/core/kotlinx-coroutines-core/test/scheduling/CoroutineSchedulerStressTest.kt
index 68e38af..683a889 100644
--- a/core/kotlinx-coroutines-core/test/scheduling/CoroutineSchedulerStressTest.kt
+++ b/core/kotlinx-coroutines-core/test/scheduling/CoroutineSchedulerStressTest.kt
@@ -78,7 +78,7 @@
 
         finishLatch.await()
 
-        require(blockingThread!! !in observedThreads)
+        require(!observedThreads.containsKey(blockingThread!!))
         validateResults()
     }