Problems with Segment Registration Module

Good afternoon,

I was trying to compute the deformation vector field between two whole brain MRI gray-scale (with segmentations) images. I tried to use the segment registration module to assist in that purpose. However, as I input the images and segmentations of the fixed and moving images and hit run, a watch icon appeared, and the following error messages are generated in the Python console:

Traceback (most recent call last):
File “/Applications/”, line 256, in onPerformRegistration
if self.logic.performRegistration():
File “/Applications/”, line 371, in performRegistration
File “/Applications/”, line 560, in createContourLabelmaps
AttributeError: ‘NoneType’ object has no attribute ‘UnRegister’
[VTK] CropVolume: Invalid input volume or ROI
[Python] Unable to access cropped moving volume
[Python] Invalid data selection
[VTK] Generic Warning: In vtkSlicerSegmentationsModuleLogic.cxx, line 408
[VTK] vtkSlicerSegmentationsModuleLogic::CreateOrientedImageDataFromVolumeNode: Invalid volume node

I have saved both the images and segmentations in nii.gz format and loaded them as volume and segmentations respectively. Should I change the input data into DICOM as in the tutorials to make it working? Thank you for the help in advance!