closing handle fails

I open a handle to a com port with
m_hPort = ::CreateFile(_T("COM1"),GENERIC_READ | GENERIC_WRITE, 0,
                       0, OPEN_EXISTING,FILE_FLAG_OVERLAPPED, 0);

Later, when I try to close the handle, the call to
CloseHandle(m_hPort) gets stuck when CloseHandle calls NtClose.

What trace can I provide here to help?

-Steve