diff --git a/config_os.h b/config_os.h
index da27de22..9703f77d 100644
--- a/config_os.h
+++ b/config_os.h
@@ -126,6 +126,7 @@
 #endif
 
 #ifdef CRYPTOPP_WIN32_AVAILABLE
+#include <winapifamily.h>
 # if !defined(WINAPI_FAMILY)
 #	define THREAD_TIMER_AVAILABLE
 # elif defined(WINAPI_FAMILY)