[R-sig-ME] [R] understanding I() in lmer formula

[Don Cohen]

Bert Gunter suggested posting this here:

 Is there a difference between I(x*y) and I(y*x) ?
I have a call to lmer that results in this complaint:
  Error in is.alpha2.subordinate * ~z.min.co.res :
  non-numeric argument to binary operator
when I change this line:
  I(is.alpha2.subordinate*z.min.co.res)+
to this:
  I(z.min.co.res*is.alpha2.subordinate)+
the complaint goes away.
I'd like to understand why.