Since Lua cannot store full 64-bit integers, this function takes a string. It is mainly aimed at usage with Player:SteamID64.

If your input is a number and not a string, it won't be networked correctly as soon as it has more than 13 digits.
This is because Lua represents numbers over 13 digits as 1e+14(100 000 000 000 000)
You can do something like this to convert it to a string: string.format("%.0f", number).
If you try to use tostring it will fail because it will create a result something like 1e+14 which doesn't work.