Obtaining a derivative of nls() SSlogis function

nls.fn <- asym/((1+exp((xmid-x.seq)/scal)))

nls.fn <- expression(asym/((1+exp((xmid-x.seq)/scal))))
D(nls.fn,"asym")

Message: 88
Date: Thu, 17 Nov 2011 12:40:59 -0900
From: Katrina Bennett <kebennett at alaska.edu>
To: r-help at r-project.org
Subject: [R] Obtaining a derivative of nls() SSlogis function
Message-ID:
	<CA+SNM83UtZMd1HDFGf2BioLPWGwhS-OWRMaHoV4yKQtvSwy4tQ at mail.gmail.com>
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Hello, I am wondering if someone can help me. I have the following function
that I derived using nls() SSlogis. I would like to find its derivative. I
thought I had done this using deriv(), but for some reason this isn't
working out for me.

Here is the function:
asym <- 84.951
xmid <- 66.90742
scal <- -6.3

x.seq <- seq(1, 153,, 153)
nls.fn <- asym/((1+exp((xmid-x.seq)/scal)))

try #1
deriv(nls.fn)
#get an Error in .Internal(deriv.default(expr, namevec, function.arg, tag,
hessian)) : 'namevec' is missing

try #2
deriv(nls.fn, namevec=c("asym", "xmid", "scal"))
#this doesn't seem to give me the expression, and the gradients are zero.

I've tried to do this with Ryacas as well, but I'm lost.

Can anyone help?

Thank you,

Katrina