diff --git a/lib/Fuzzer/FuzzerUtil.cpp b/lib/Fuzzer/FuzzerUtil.cpp
index 5f76ddc3678..f5a77737449 100644
--- a/lib/Fuzzer/FuzzerUtil.cpp
+++ b/lib/Fuzzer/FuzzerUtil.cpp
@@ -195,7 +195,15 @@ void PrintPC(const char *SymbolizedFMT, const char *FallbackFMT, uintptr_t PC) {
     Printf(FallbackFMT, PC);
 }
 
-unsigned NumberOfCpuCores() { return hardware_concurrency(); }
+unsigned NumberOfCpuCores() {
+  unsigned N = std::thread::hardware_concurrency();
+  if (!N) {
+    Printf("WARNING: std::thread::hardware_concurrency not well defined for "
+           "your platform. Assuming CPU count of 1.\n");
+    N = 1;
+  }
+  return N;
+}
 
 size_t SimpleFastHash(const uint8_t *Data, size_t Size) {
   size_t Res = 0;