added include <iostream>. Closes #674
diff --git a/src/compiler/ruby_generator_map-inl.h b/src/compiler/ruby_generator_map-inl.h
index 523f57b..a86342e 100644
--- a/src/compiler/ruby_generator_map-inl.h
+++ b/src/compiler/ruby_generator_map-inl.h
@@ -34,6 +34,7 @@
 #ifndef NET_GRPC_COMPILER_RUBY_GENERATOR_MAP_INL_H_
 #define NET_GRPC_COMPILER_RUBY_GENERATOR_MAP_INL_H_
 
+#include <iostream>
 #include <initializer_list>
 #include <map>
 #include <ostream>  // NOLINT