Using lm coefficients in polyroot()

Dear useRs,

I need to compute zero of polynomial function fitted by lm. For example 
if I fit cubic equation by fit=lm(y~x+I(x^2)+i(x^3)) I can do it simply 
by polyroot(fit$coefficients). But, if I fit polynomial of higher order 
and optimize it by stepAIC, I get of course some coefficients removed. 
Then, if i have model

y ~ I(x^2) + I(x^4)

i cannot call polyroot in such way, because there is a need to call 
polyroot(c(0,0,fit$coefficients[1],0,fit$coefficients[2]).

Is there any method to do it automagically? I would like to write small 
function solving polynomial optimized by stepAIC, regardless of missing 
terms.

Sincerely

Lukasz Komsta
Department of Medicinal Chemistry
Medical University of Lublin
Jaczewskiego 4, 20-090 Lublin, Poland
Fax +48 81 7425165