diff --git a/extra/binary_as_string_codec.go b/extra/binary_as_string_codec.go
index 57d0ae5b..13bc97ea 100644
--- a/extra/binary_as_string_codec.go
+++ b/extra/binary_as_string_codec.go
@@ -180,9 +180,9 @@ func readHex(iter *jsoniter.Iterator, b1, b2 byte) byte {
 	}
 	ret *= 16
 	if b2 >= '0' && b2 <= '9' {
-		ret = b2 - '0'
+		ret += b2 - '0'
 	} else if b2 >= 'a' && b2 <= 'f' {
-		ret = b2 - 'a' + 10
+		ret += b2 - 'a' + 10
 	} else {
 		iter.ReportError("read hex", "expects 0~9 or a~f, but found "+string([]byte{b2}))
 		return 0
diff --git a/extra/binary_as_string_codec_test.go b/extra/binary_as_string_codec_test.go
index a00479e6..30d6c4d7 100644
--- a/extra/binary_as_string_codec_test.go
+++ b/extra/binary_as_string_codec_test.go
@@ -22,11 +22,11 @@ func TestBinaryAsStringCodec(t *testing.T) {
 	})
 	t.Run("non safe set", func(t *testing.T) {
 		should := require.New(t)
-		output, err := jsoniter.Marshal([]byte{1, 2, 3, 15})
+		output, err := jsoniter.Marshal([]byte{1, 2, 3, 23})
 		should.NoError(err)
-		should.Equal(`"\\x01\\x02\\x03\\x0f"`, string(output))
+		should.Equal(`"\\x01\\x02\\x03\\x17"`, string(output))
 		var val []byte
 		should.NoError(jsoniter.Unmarshal(output, &val))
-		should.Equal([]byte{1, 2, 3, 15}, val)
+		should.Equal([]byte{1, 2, 3, 23}, val)
 	})
 }