mirtk padding failure for tissue labels

[pkashya]

Error messages from the .log and .err respectively:

mirtk padding segmentations/{99988811}.nii-session1_tissue_labels.nii.gz segment
ations/{99988811}.nii-session1_tissue_labels.nii.gz masks/{99988811}.nii-session
1-labels.nii.gz 2 1 4 0 : failed

IOError: Cifst
ream::Open: Failed to open file segmentations/{99988811}.nii-session1_tissue_lab
els.nii.gz
Error: padding command returned non-zero exit status 1

I have verified that the tissue_labels file exists and has the right labels...
The padding sub-routine works fine until this step.
Does anyone know how to debug this or has experienced this before?