diff --git a/sm9/bn256/curve.go b/sm9/bn256/curve.go
index 122920f..61c4133 100644
--- a/sm9/bn256/curve.go
+++ b/sm9/bn256/curve.go
@@ -133,7 +133,7 @@ func (c *curvePoint) Add(a, b *curvePoint) {
 
 	gfpSub(t, s2, s1)
 
-	if h.Equal(zero) == 1 && t.Equal(one) == 1 {
+	if h.Equal(zero) == 1 && t.Equal(zero) == 1 {
 		c.Double(a)
 		return
 	}
diff --git a/sm9/bn256/g1_test.go b/sm9/bn256/g1_test.go
index acc8bb8..25f990f 100644
--- a/sm9/bn256/g1_test.go
+++ b/sm9/bn256/g1_test.go
@@ -24,6 +24,16 @@ func TestG1AddNeg(t *testing.T) {
 	}
 }
 
+func TestG1AddSame(t *testing.T) {
+	g1, g2 := &G1{}, &G1{}
+	g1.Add(Gen1, Gen1)
+	g2.Double(Gen1)
+
+	if !g1.Equal(g2) {
+		t.Fail()
+	}
+}
+
 type g1BaseMultTest struct {
 	k string
 }