ERROR with dwifslpreproc : "DWI volumes could not be classified into b-value shells; gradient encoding may not represent a HARDI sequence"

Dear MRtrix experts,

I would like to create the connectome and meanlength matrix from my data, however when I run the command
dwifslpreproc sub-01_den.mif sub-01_den_preproc.mif -nocleanup -rpe_none -pe_dir PA -eddy_options " --slm=linear "
I get the following error:
[ERROR] DWI volumes could not be classified into b-value shells; gradient encoding may not represent a HARDI sequence

Looking at previous posts I run the command
mrinfo sub-01_dwi.nii.gz -fslgrad sub-01_dwi.bvec sub-01_dwi.bval -dwgrad
and got the following reply:
0 0 0 0
0.996803021311441 -0.0492691877250133 0.0628989971724458 500
0.0418844791812721 0.992625217661593 0.113757934518827 500
-0.0680398877645681 -0.110759753876823 0.99151543134443 500
0.996803021311441 -0.0492691877250133 0.0628989971724458 1000
0.0418844791812727 0.992625217661594 0.113757934518818 1000
-0.0680398877645681 -0.110759753876823 0.99151543134443 1000

The following are the commands I have passed since the beginning:
mrconvert sub-01_dwi.nii.gz sub-01_dwi.mif -fslgrad sub-01_dwi.bvec sub-01_dwi.bval

dwidenoise sub-01_dwi.mif sub-01_den.mif -noise noise.mif

dwifslpreproc sub-01_den.mif sub-01_den_preproc.mif -nocleanup -rpe_none -pe_dir PA -eddy_options " --slm=linear "

Adding the option --data_is_shelled had no effect.

I would be grateful if you could help me!

Thank you very much,

Miche