diff --git a/tests/unit/core/test-core-calc.cpp b/tests/unit/core/test-core-calc.cpp
index da6e35aab..8a0d4c9b1 100644
--- a/tests/unit/core/test-core-calc.cpp
+++ b/tests/unit/core/test-core-calc.cpp
@@ -25,6 +25,7 @@ extern "C"
 {
 #include <stdio.h>
 #include <string.h>
+#include <locale.h>
 #include "src/core/wee-arraylist.h"
 #include "src/core/wee-calc.h"