  We investigate the metallicity properties of host galaxies of long Gamma-ray
Bursts (GRBs) in the light of the Fundamental Metallicity Relation (FMR), the
tight dependence of metallicity on mass and SFR recently discovered for SDSS
galaxies with stellar masses above 10^9.2 Msun. As most of the GRB hosts have
masses below this limit, the FMR can only be used after an extension towards
lower masses. At this aim, we study the FMR for galaxies with masses down to
~10^8.3 Msun, finding that the FMR does extend smoothly at lower masses, albeit
with a much larger scatter. We then compare the resulting FMR with the
metallicity properties of 18 host galaxies of long GRBs. While the GRB hosts
show a systematic offset with respect to the mass-metallicity relation, they
are fully consistent with the FMR. This shows that the difference with the
mass-metallicity relation is due to higher than average SFRs, and that GRBs
with optical afterglows do not preferentially select low-metallicity hosts
among the star-forming galaxies. The apparent low metallicity is therefore a
consequence of the occurrence of long GRB in low mass, actively star-forming
galaxies, known to dominate the current cosmic SFR.
