ITK(in ANTs) occurs error : no orthonormal definition found

Question

ANTs open source provides transform functions that can move(frequently referred as normalizing or registration in neuroscience) one's brain MR image to other's brain MR image. But I got an error below : not all MR images but some MR images make below error. There NEVER exist any differences when I opened those MR images with image viewer like ITK-snap. It seems centain MR images that occurs those error must have mathematical or algebraic differences with images that does not occur error. How can I find out what the problem is?

Error message:

/opt/ANTs/bin/antsRegistrationSyNQuick.sh: line 464:

[[: _MR.nii: syntax error: invalid arithmetic operator (error token is ".nii")

antsRegistration call:

/opt/ANTs/bin//antsRegistration --verbose 1 --dimensionality 3 --float 0 --collapse-output-transforms 1 --output [ T1xPET,T1xPETWarped.nii.gz,T1xPETInverseWarped.nii.gz ]
--interpolation Linear --use-histogram-matching 0 --winsorize-image-intensities [ 0.005,0.995 ] --initial-moving-transform [ _MR.nii,_FBB.nii,1 ] --transform Rigid[ 0.1 ] --metric MI[ _MR.nii,_FBB.nii,1,32,Regular,0.25 ] --convergence [ 1000x500x250x0,1e-6,10 ] --shrink-factors 8x4x2x1 --smoothing-sigmas 3x2x1x0vox

Output:

All_Command_lines_OK Using double precision for computations. Exception caught during reference file reading

itk::ExceptionObject (0x5559bfd38530) Location: "unknown" File: /home/nuc/Desktop/a/build/ITKv5/Modules/IO/NIFTI/src/itkNiftiImageIO.cxx Line: 1980

Description: ITK ERROR: ITK only supports orthonormal direction cosines.  No orthonormal definition found!

file _MR.nii Exception Object caught:

itk::ExceptionObject (0x5559bfd38530) Location: "unknown" File: /home/nuc/Desktop/a/build/staging/include/ITK-5.2/itkCenteredTransformInitializer.hxx Line: 40

Description: ITK ERROR: CenteredTransformInitializer(0x5559bfd22e10): Fixed Image has not been set