diff --git a/raft/log.go b/raft/log.go
index 83dcb662c..e626385d5 100644
--- a/raft/log.go
+++ b/raft/log.go
@@ -339,7 +339,7 @@ func (l *raftLog) mustCheckOutOfBounds(lo, hi uint64) error {
 		return ErrCompacted
 	}
 
-	length := l.lastIndex() - fi + 1
+	length := l.lastIndex() + 1 - fi
 	if lo < fi || hi > fi+length {
 		l.logger.Panicf("slice[%d,%d) out of bound [%d,%d]", lo, hi, fi, l.lastIndex())
 	}