Nonpositive F statistic

Steve Bousquin (
Sat, 31 Jan 1998 22:48:03 -0700

Can anyone tell me what the following message means and what caused it
(from a stepwise AIC regression (lm) :

Warning messages:
One or more nonpositive parameters in: pf(x$fstatistic[1],

I take it it refers to the F-statistic, but I cannot find these
variables (?) in the lm output or attributes and . . . and why do all
statistics show as "NA" ?

I could send more detail if interested . . .

Steve Bousquin