diff --git a/include/ruby/missing.h b/include/ruby/missing.h
index 1d7d2b8a82..aaf89f9664 100644
--- a/include/ruby/missing.h
+++ b/include/ruby/missing.h
@@ -18,6 +18,8 @@ extern "C" {
 #endif
 #endif
 
+#define __STDC_WANT_LIB_EXT1__ 1
+
 #include "ruby/config.h"
 #include <stddef.h>
 #include <math.h> /* for INFINITY and NAN */