  The determination of the redshift of extragalactic gamma ray sources is of
fundamental importance for the interpretation of their emission models and
extragalactic population studies, AGN classification schemes, and to study the
interaction with the extragalactic background light. Here we focus on the BL
Lac S2 0109+22, recently detected at very high energies. We obtained a high
signal-to-noise optical spectrum at Gran Telescopio Canarias for the source
over the spectral range 4000-10000 Angstrom, to search for spectral features of
the host galaxy and/or of the nuclear source and it was compared with previous
observations in order to confirm and/or dispute its redshift determination. We
find the S2 0109+22 optical spectrum is well represented by a power law
continuum without any absorption or emission feature of Equivalent Width >0.1
Angstrom. Based on the new optical spectrum we are able to set a redshift lower
limit z>0.35, assuming the source is hosted by a massive elliptical galaxy
typical for this class of sources. The redshift z=0.265 proposed in the
literature for this object is most probably referred to a r=18.3 galaxy at 15"
from S2 0109. This galaxy could belong to a group of faint galaxies located at
z~0.26.
