diff --git a/epoch.go b/epoch.go
index 0a2c184..6ec6602 100644
--- a/epoch.go
+++ b/epoch.go
@@ -11,7 +11,7 @@ const (
 )
 
 func IsExpired(prev StatePeriod, cur StatePeriod) bool {
-	return cur-prev >= NumActiveEpochs
+	return cur >= prev+NumActiveEpochs // TODO(weiihann): deal with overflow
 }
 
 func StatePeriodFromBytes(b []byte) StatePeriod {