Why am I getting ft_defaults error (line 88) ?

I am using fieldtrip to do electrode reconstructions. When I run

cfg = [];
cfg.filename = ['SubjectUCI29', '_MRI_acpc'];
cfg.filetype = 'nifti';
cfg.parameter = 'anatomy';
ft_volumewrite(cfg, mri_acpc);  %%%This line is where the error pops up because ft_default and ft_defaults are used in ft_volumewrite

to write the preprocessed anatomical MRI out a to file, I get an error in ft_defaults line 88 (the line below):

ft_default = mergeconfig(ft_default, prefs);  

[Benjamin Thompson]

You are asking about errors in functions not part of MATLAB so if these functions are your code you will need to post it to get a good answer.