from_base64 UNDEFINED BEHAVIOR for invalid base64 string

[ROCKFAL1]

Compiler: MSVC
boost version: 1.86.0

For any string with size() % 4 == 1 from_base64 function increments past-of-end iterator
Is this behavior expected? If this behavior is expected, then why doesn't the library provide functions for base64 validation?

MRE:

#include <string>
#include <vector>
#include <algorithm>
#include <iterator>

#include <boost/archive/iterators/base64_from_binary.hpp>
#include <boost/archive/iterators/binary_from_base64.hpp>
#include <boost/archive/iterators/transform_width.hpp>

template <typename OutIt>
OutIt from_base64(const char* pstr, size_t size, OutIt out)
{
  using namespace boost::archive::iterators;

  std::vector<char> buffer; // Just for debugging 
  std::copy_n(pstr, size, std::back_inserter(buffer));

  using It = transform_width<binary_from_base64<std::vector<char>::iterator>, 8, 6>;
  return  std::copy(It(buffer.begin()), It(buffer.end()), out);
}

const std::string base64str= "MTIzN"; // base64("1234567890") == "MTIzNDU2Nzg5MA=="
std::string result;
from_base64(base64str.c_str(), base64str.size(), std::back_inserter(result));
 // Here i'm expecting smth like `result == "123"`, or `from_base64` throw exception (that i can wrap and return only valid part - "123")