diff --git a/sm9/bn256/gfp.go b/sm9/bn256/gfp.go
index 10291de..c427bbc 100644
--- a/sm9/bn256/gfp.go
+++ b/sm9/bn256/gfp.go
@@ -150,7 +150,7 @@ func (e *gfP) Unmarshal(in []byte) error {
 }
 
 func montEncode(c, a *gfP) { gfpMul(c, a, r2) }
-func montDecode(c, a *gfP) { gfpFromMont(c, a) }
+func montDecode(c, a *gfP) { gfpMul(c, a, &gfP{1}) }
 
 // cmovznzU64 is a single-word conditional move.
 //