diff --git a/bindings/ruby/rbsigar.c b/bindings/ruby/rbsigar.c
index dde9a78e..f1372b1f 100644
--- a/bindings/ruby/rbsigar.c
+++ b/bindings/ruby/rbsigar.c
@@ -26,11 +26,11 @@
 #define NUM2PID NUM2UINT
 
 #ifndef RSTRING_PTR
-#define RSTRING_PTR(s) (s)->ptr
+#define RSTRING_PTR(s) RSTRING(s)->ptr
 #endif
 
 #ifndef RSTRING_LEN
-#define RSTRING_LEN(s) (s)->len
+#define RSTRING_LEN(s) RSTRING(s)->len
 #endif
 
 static sigar_t *rb_sigar_get(VALUE obj)