diff --git a/sink_file.h b/sink_file.h
index e2c69d8..ebec522 100644
--- a/sink_file.h
+++ b/sink_file.h
@@ -33,7 +33,7 @@
 
  private:
   // The sink() function used to write data.
-  const sink_func& sink_;
+  const sink_func sink_;
 };
 
 }  // namespace bsdiff