Why is npos max 63-bit number instead of like std::string?

This isn't a bug report but I'm wondering why? Since they're not the same I can't use this in std::string's place as std::string::npos is the maximum 64-bit unsigned integer, and it breaks my code. Was there a reason why it wasn't simply made the same? Is there any way of changing it to be the same so I can use it?