diff --git a/llvm/lib/Fuzzer/FuzzerLoop.cpp b/llvm/lib/Fuzzer/FuzzerLoop.cpp
index 63dd731557275..861ab13e686f3 100644
--- a/llvm/lib/Fuzzer/FuzzerLoop.cpp
+++ b/llvm/lib/Fuzzer/FuzzerLoop.cpp
@@ -296,7 +296,10 @@ void Fuzzer::InterruptCallback() {
 NO_SANITIZE_MEMORY
 void Fuzzer::AlarmCallback() {
   assert(Options.UnitTimeoutSec > 0);
+  // In Windows Alarm callback is executed by a different thread.
+#if !LIBFUZZER_WINDOWS
   if (!InFuzzingThread()) return;
+#endif
   if (!RunningCB)
     return; // We have not started running units yet.
   size_t Seconds =