For the the following alignment we can't calculate the background model and phyml gives this strange warning and asks for a user input "type any key to exit" which is super annoying.
a score=0
s mm9.chr2 94496640 4 + 0 GAAA
s hg18.chr11 91392746 4 - 0 GAAA
s panTro2.chr11 90665046 4 - 0 GAAA
s rheMac2.chr14 28971164 4 + 0 GAAA
s canFam2.chr18 29504980 4 + 0 GAAA