classification - Empty signature file after i.gensig in GRASS

I'm trying to make a supervised classification in Grass 6.4.3 under Linuxmint 16.

After digitizing the training areas, I transformed them from vector to raster. Then I've done the following: just in case: g.region rast=train_area_2013_03_05_raster@PERMANENT,2013_03_05_toar4@PERMANENT

then

i.group group=group subgroup=sub_group input=2013_03_05_toar4@PERMANENT,2013_03_05_toar5@PERMANENT,2013_03_05_toar3@PERMANENT

and

i.gensig trainingmap=train_area_2013_03_05_raster@PERMANENT group=group@PERMANENT subgroup=sub_group signaturefile=superv_class

but the resulting signature file, only contains one character: "#".

The output of i.gensig in the console is the following:

Finding training classes...
3 classes found
Calculating class means...
Calculating class covariance matrices...
Signature 1 not invertible
Signature 2 not invertible
Signature 3 not invertible
i.gensig complete.

Obviously if I try: i.maxlik group=group@PERMANENT subgroup=sub_group sigfile=superv_class class=results_class

I obtain the following error message:

ERROR: Unable to read signature file

I've been searching, but I couldn't find the solution.

I've tried: this Problem running i.gensigset in Grass. Any ideas?

My problem is similar to this: i.maxlik cannot read i.class output signature file. But here, there's no solution suggested. As in this case, I also used i.cluster to generate an unsupervised signature file and i.maxlik reads this perfectly well.

Here there's another similar question, but again, with no solution...

What does Signature X not invertible means?

Any idea? Thanks for your help!