[Freesurfer] mri_convert error

Hello

When I tried to run mri_convert brain.mgz to get brain.nii to get betted brain. nii, I received an error message following next:

mri_convert -it mgz -i brain.mgz -ot nii -o brain.nii.gz $Id: mri_convert.c,v 1.179.2.7 2012/09/05 21:55:16 mreuter Exp $ reading from brain.mgz... TR=2530.00, TE=1.64, TI=1200.00, flip angle=7.00 i_ras = (-1, 7.45058e-08, -7.72998e-08) j_ras = (7.35745e-08, 4.30737e-09, -1) k_ras = (1.49012e-08, 1, 1.80444e-09) writing to brain.nii.gz... niiWrite(): error opening file brain.nii.gz ERROR: failure writing brain.nii.gz as volume type 24

What is the problem to convert brain.mgz into brain.nii? How can I fix the error?

Thank you.

Jung Mi