PVS-Studio warning V510 for smtp_ssl

bool CSmtp::ConnectRemoteServer(....)
{
  ...
  std::string encoded_login =
    base64_encode(ustrLogin, strlen(SendBuf));
  ...
  sprintf(SendBuf, "AUTH PLAIN %s",
    encoded_login);
  ...
}

This is what should have been written here: sprintf(SendBuf, "AUTH PLAIN %s", encoded_login.c_str());

This suspicious code was found in smtp_ssl project by PVS-Studio static code analyzer.
Warning message is:
V510 The 'sprintf' function is not expected to receive class-type variable as third actual argument. CSmtp csmtp.cpp 809

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/