diff --git a/src/cpp/util/string_ref.cc b/src/cpp/util/string_ref.cc
index 9adc0920135..c42033f61fe 100644
--- a/src/cpp/util/string_ref.cc
+++ b/src/cpp/util/string_ref.cc
@@ -36,6 +36,7 @@
 #include <string.h>
 
 #include <algorithm>
+#include <iostream>
 
 namespace grpc {