diff --git a/prefs.cc b/prefs.cc
index 223fc9b..1bde4cd 100644
--- a/prefs.cc
+++ b/prefs.cc
@@ -19,6 +19,7 @@
 const char kPrefsLastActivePingDay[] = "last-active-ping-day";
 const char kPrefsLastRollCallPingDay[] = "last-roll-call-ping-day";
 const char kPrefsManifestMetadataSize[] = "manifest-metadata-size";
+const char kPrefsPreviousVersion[] = "previous-version";
 const char kPrefsResumedUpdateFailures[] = "resumed-update-failures";
 const char kPrefsUpdateCheckResponseHash[] = "update-check-response-hash";
 const char kPrefsUpdateStateNextDataOffset[] = "update-state-next-data-offset";