pyomo wrong guess_format()

run.sh

~/work/pyomo/pyomo$ svn up
Updating '.':
At revision 11187.

~/work/pyomo/pyomo$ cd ~/tmp
~/tmp$ python write.py
~/tmp$ diff modelb.mps model.lp # no difference, same file

write.py

from pyomo.environ import *
from pyomo.opt import ProblemFormat
def create_model():
    model = ConcreteModel()
    model.x = Var()
    model.o = Objective(expr=model.x)
    model.c = Constraint(expr=model.x >= 1)
    model.x.set_value(1.0)
    return model
    
model = create_model()
model.write('modela.mps', format=ProblemFormat.mps)
model.write('modelb.mps')
model.write('model.lp', format=ProblemFormat.cpxlp)