Index: lib/Fuzzer/FuzzerLoop.cpp
===================================================================
--- lib/Fuzzer/FuzzerLoop.cpp
+++ lib/Fuzzer/FuzzerLoop.cpp
@@ -288,7 +288,10 @@
 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 =