Using smbc_setxattr to set the owner of a file only works when using numeric data input (SID)

E.g
Setting GROUP with Name

strcpy( pBuffer, "BUILTIN\\Users" );
nReturn = pContext->setxattr( pContext, url.c_str(), "system.nt_sec_desc.group+", pBuffer, strlen(pBuffer), 0 );
cout << "nReturn: " << nReturn << ", errno: " << errno << endl;
----------------------------
nReturn: -1, errno: 22
FAILS with Invalid argument

Setting GROUP with SID
strcpy( pBuffer, "S-1-5-32-545" );
nReturn = pContext->setxattr( pContext, url.c_str(), "system.nt_sec_desc.group", pBuffer, strlen(pBuffer), 0 );
cout << "nReturn: " << nReturn << ", errno: " << errno << endl;
---------------------
nReturn: 0, errno: 0
SUCCESS

Cheers,
Henke