diff --git a/lib/rb/ext/memory_buffer.c b/lib/rb/ext/memory_buffer.c
index 3a9c101..74efa2c 100644
--- a/lib/rb/ext/memory_buffer.c
+++ b/lib/rb/ext/memory_buffer.c
@@ -54,7 +54,7 @@
     index = 0;
   }
 
-  if (RSTRING(data)->len < length) {
+  if (RSTRING_LEN(data) < length) {
     rb_raise(rb_eEOFError, "Not enough bytes remain in memory buffer");
   }