diff --git a/src/cpp/util/string_ref.cc b/src/cpp/util/string_ref.cc
index 9adc092013525fa758cd2b63dcf680f233d00fd5..c42033f61fe6f29c0851517c1beb1f20b3ead7cb 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 {