Index: llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
===================================================================
--- llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
+++ llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
@@ -296,7 +296,10 @@
 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 =