diff --git a/util/mutex.h b/util/mutex.h
index 5dc1693..e2a8715 100644
--- a/util/mutex.h
+++ b/util/mutex.h
@@ -11,7 +11,10 @@
  */
 
 #ifdef _WIN32
+// Requires Windows Vista or Windows Server 2008 at minimum.
+#if defined(WINVER) && WINVER >= 0x0600
 #define MUTEX_IS_WIN32_SRWLOCK
+#endif
 #else
 #ifndef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200809L