call step inside a function

I am wondering why my function works fine in R1.7.1 and R1.8.1 but not in 
R1.9.0. I thought it's an environment related problem but I can't solve it.

data

weta jd
1     1  4
2     2 13
3     2 13
4     6  4
5     1  3
6     1  7
7     2 10
8     3 10
9     1  8
10    1  8
11    3  6
12    1  9
13    1  5
14    1  1
15    3 13
16    1  2
17    2  2
18    7 11
19    1  3
20    5  4
21    1  6
22    4  9
23    1  6
24    4  5
25    5  5
26    2  6

program

function(dataset)
{
        tmp<-glm(weta~1, family=poisson, data=dataset)
        tmp.f<-step(tmp,~.+jd)    
}

When I run program(data) in 1.9.0, an error message appears:

Error in model.frame.default(formula = WETA ~ jd, data = dataset, 
drop.unused.levels = TRUE) :
        Object "dataset" not found


Thanks for help in advance!

Weihong Zeng
University of Albert