mfc and string question

void CStrTest2Dlg::OnStrT1()
{
// TODO: Add your control notification handler code here
string str1, str2;

str1 = "part 1";
str2 = "part 2";
m_StrT1 = str1 + str2;
UpdateData(FALSE);

}

why this is giving me
C:\Documents\Programming\c++\Projects\StrTest2\StrTest2Dlg.cpp(104) : error C2679: binary '=' : no operator defined which takes a right-hand operand of type 'class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >' (o
r there is no acceptable conversion)
Error executing cl.exe.

how can i make this work?