[R] Formula with in memory data.frame

[rkevinburton at charter.net]

I have an array of data.frame(s) that I would like to smooth with loess one at a time. the array is master and the two variable that I am interested in is Period and Quantity. So my first attempt at calling loess is:

loess(Quantity ~ Period, master[[i]])

But I get the following error:

Error: NA/NaN/Inf in foreign function call (arg 2)
In addition: Warning message:
NAs introduced by coercion 

I did a str on the array element str(master[[j]]):

'data.frame':	58 obs. of  3 variables:
 Factor w/ 41 levels "10\" Plates",..: 1 1 1 1 1 1 1 1 1 1 ...
 $ Period  : POSIXct, format: "0001-01-20" "0002-01-20" ...
 $ Quantity: int  0 0 0 0 0 0 0 0 0 0 ...

So am I doing something wrong?

Thank you.

Kevin