diff --git a/src/frames.h b/src/frames.h
index 19860ad..8cbbc62 100644
--- a/src/frames.h
+++ b/src/frames.h
@@ -589,6 +589,9 @@
  public:
   StackTraceFrameIterator();
   void Advance();
+
+ private:
+  bool IsValidFrame();
 };