diff --git a/src/thread.cpp b/src/thread.cpp
index b2bd07e9d..184d81e6b 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -71,7 +71,7 @@ thread::hardware_concurrency() _NOEXCEPT
     // does not have a definite limit.
     if (result == -1)
         return 0;
-    return result;
+    return static_cast<unsigned>(result);
 #else  // defined(CTL_HW) && defined(HW_NCPU)
     // TODO: grovel through /proc or check cpuid on x86 and similar
     // instructions on other architectures.