diff --git a/action_mock.h b/action_mock.h
index 07f435e..505fbf5 100644
--- a/action_mock.h
+++ b/action_mock.h
@@ -5,6 +5,8 @@
 #ifndef CHROMEOS_PLATFORM_UPDATE_ENGINE_ACTION_MOCK_H_
 #define CHROMEOS_PLATFORM_UPDATE_ENGINE_ACTION_MOCK_H_
 
+#include <string>
+
 #include <gmock/gmock.h>
 
 #include "update_engine/action.h"