diff --git a/ext/mbstring/oniguruma/win32/config.h b/ext/mbstring/oniguruma/win32/config.h
index bdbdaf25c1f..b2afbe960c0 100644
--- a/ext/mbstring/oniguruma/win32/config.h
+++ b/ext/mbstring/oniguruma/win32/config.h
@@ -15,6 +15,11 @@
 #define SIZEOF_VOIDP 4
 #define SIZEOF_FLOAT 4
 #define SIZEOF_DOUBLE 8
+#ifdef _WIN64
+#define SIZEOF_SIZE_T 8
+#else
+#define SIZEOF_SIZE_T 4
+#endif
 #define HAVE_PROTOTYPES 1
 #define TOKEN_PASTE(x,y) x##y
 #define HAVE_STDARG_PROTOTYPES 1