diff --git a/tests/utils.h b/tests/utils.h
index 8b7e8ec..6393d85 100644
--- a/tests/utils.h
+++ b/tests/utils.h
@@ -20,6 +20,8 @@
 #include "../src/gmio_core/internal/string_utils.h"
 #include "../src/gmio_core/internal/convert.h"
 
+#include <stddef.h>
+
 /*! Does \p a and \p b compares equals by ULP (Units in the Last Place) ?
  *
  *  ULP = spacing between floating-point numbers