diff --git a/cpp/src/phonenumbers/regexp_adapter_re2.cc b/cpp/src/phonenumbers/regexp_adapter_re2.cc
index bb54223..398f369 100644
--- a/cpp/src/phonenumbers/regexp_adapter_re2.cc
+++ b/cpp/src/phonenumbers/regexp_adapter_re2.cc
@@ -31,6 +31,8 @@
 namespace i18n {
 namespace phonenumbers {
 
+using re2::StringPiece;
+
 // Implementation of RegExpInput abstract class.
 class RE2RegExpInput : public RegExpInput {
  public: