[R] model fine, predict gives an error

[Antje Niederlein]

Hi there,

I try to model some dose response curves (drc-package). In most cases
it is fine but now I got some data which produces me the following
error:

load("drmData.RData")
library(drc)

drmObj <- drm(value ~ concentration, cmpd_respvar, data = drmData, fct = LL.4())
predict(drmObj)

>> Error in chol.default(0.99 * object$fit$hessian + 0.01 * diag(dim(object$fit$hessian)[1])) :
  the leading minor of order 2 is not positive definite
Error in resultMat[, 2] <- estSE : replacement has length zero

Can anybody explain to me why the calculation of the dose response
model works but the prediction fails? Anything I can do in this case?

Antje