Issue with convert value string into std::string and vice versa

[tuongbuimtt]

Sometimes my output JSON has an invalid utf-8 string.
It seems to appear in a random position. My JSON has many nested fields, and adding one by one.
I mainly write values from std::string.
Do you have any advice for my case?

"sid":"1420\u0000\u0000\u0000\u00003649932"
the correct string is 14203649932

Thanks.