When setting MathsCommand().inputs.out_file argument, a Trait error is thrown because of the exists=True:

I think this doesn't make any sense, because out_file is to be created by MathsCommand().
In my case, I just want to get rid of SPM nan's in an .nii file, but I also don't want fsl_maths to append a suffix to the filename - this is why I also provide out_file to MathsCommand().inputs.
A short example:

nan2zero = pe.MapNode(interface=MathsCommand(), name='nan2zero', iterfield=['in_file', 'out_file'])
nan2zero.inputs.nan2zeros = True