from feedback buffer to gluUnProject

from feedback buffer i got the xMax, xMin, yMax
yMin,zMax, zMin values
I want to use gluUnproject to turn them into 3D coordinate, but i failed!the result is not right!
gluUnProject((GLdouble)m_xMin,
(GLdouble)m_yMin,
(GLdouble)m_zMax,
dModelMatrix,
dProjMatrix,
iViewPort,
&ObjectX,
&ObjectY,
&ObjectZ);
???

i use the glutLookAt function in my program!
is there any thing about it?