nls

HI,
i have some prob when i try to use nls().
my data is 1D vector, I tried to use a polynomial function(order is 3) to
fit it.
the data series is stored in x.
the a0, a1, a2, a3 below is coefficient, which i  hope i can get from
calls "nls"

z <-  nls(  ~ a0 + a1 * x + a2 * x * x + a3 * x * x * x, data = x )

Error in match.call(definition, call, expand.dots) :
	.Primitive... is not a function

Any idea on what is the problem?

many thanks,
yan