diff --git a/prefs_mock.h b/prefs_mock.h
index 5a56ba0..36efd40 100644
--- a/prefs_mock.h
+++ b/prefs_mock.h
@@ -5,6 +5,8 @@
 #ifndef CHROMEOS_PLATFORM_UPDATE_ENGINE_PREFS_MOCK_H_
 #define CHROMEOS_PLATFORM_UPDATE_ENGINE_PREFS_MOCK_H_
 
+#include <string>
+
 #include "gmock/gmock.h"
 #include "update_engine/constants.h"
 #include "update_engine/prefs_interface.h"