diff --git a/lib/rb/ext/memory_buffer.c b/lib/rb/ext/memory_buffer.c
index bf11d91..e37c787 100644
--- a/lib/rb/ext/memory_buffer.c
+++ b/lib/rb/ext/memory_buffer.c
@@ -49,7 +49,7 @@
     index = RSTRING(buf)->len;
   }
   if (index >= GARBAGE_BUFFER_SIZE) {
-    rb_ivar_set(self, buf_ivar_id, rb_funcall(buf, slice_method_id, 2, INT2FIX(index), INT2FIX(-1)));
+    rb_ivar_set(self, buf_ivar_id, rb_funcall(buf, slice_method_id, 2, INT2FIX(index), INT2FIX(RSTRING(buf)->len - 1)));
     index = 0;
   }