diff --git a/mysqldef/bit.go b/mysqldef/bit.go
index 59a286bc2b..985c5b1d58 100644
--- a/mysqldef/bit.go
+++ b/mysqldef/bit.go
@@ -48,7 +48,7 @@ func (b Bit) ToString() string {
 	byteSize := (b.Width + 7) / 8
 	buf := make([]byte, byteSize)
 	for i := byteSize - 1; i >= 0; i-- {
-		buf[len(buf)-i-1] = byte(b.Value >> uint(i*8))
+		buf[byteSize-i-1] = byte(b.Value >> uint(i*8))
 	}
 	return string(buf)
 }