diff --git a/format.h b/format.h
index 9e26d1bce76f2..97a67059dbd0f 100644
--- a/format.h
+++ b/format.h
@@ -977,6 +977,7 @@ template <typename T>
 T &get();
 
 struct DummyStream : std::ostream {
+  DummyStream();
   // Hide all operator<< overloads from std::ostream.
   void operator<<(Null<>);
 };