diff --git a/sm2/util.go b/sm2/util.go
index 927ec44..c52b917 100644
--- a/sm2/util.go
+++ b/sm2/util.go
@@ -12,7 +12,7 @@ import (
 func toBytes(curve elliptic.Curve, value *big.Int) []byte {
 	byteLen := (curve.Params().BitSize + 7) >> 3
 	result := make([]byte, byteLen)
-	value.FillBytes(result[:])
+	value.FillBytes(result)
 	return result
 }