diff --git a/src/ecs-test.c b/src/ecs-test.c
index 290b0f79a..a194280f1 100644
--- a/src/ecs-test.c
+++ b/src/ecs-test.c
@@ -26,6 +26,7 @@
 
 #undef NDEBUG
 #include <assert.h>
+#include <locale.h>
 #include <stdio.h>
 
 #include "global.h"