diff --git a/common/constants.h b/common/constants.h
index 64447ce..68f720d 100644
--- a/common/constants.h
+++ b/common/constants.h
@@ -76,6 +76,7 @@
 extern const char kPrefsLastFp[];
 extern const char kPrefsPostInstallSucceeded[];
 extern const char kPrefsPreviousVersion[];
+extern const char kPrefsPreviousSlot[];
 extern const char kPrefsResumedUpdateFailures[];
 extern const char kPrefsRollbackHappened[];
 extern const char kPrefsRollbackVersion[];