diff --git a/constants.h b/constants.h
index 59e3737..3a74156 100644
--- a/constants.h
+++ b/constants.h
@@ -27,6 +27,7 @@
 extern const char kStatefulPartition[];
 
 // Constants related to preferences.
+extern const char kPrefsAttemptInProgress[];
 extern const char kPrefsBackoffExpiryTime[];
 extern const char kPrefsCertificateReportToSendDownload[];
 extern const char kPrefsCertificateReportToSendUpdate[];