Eu quero usaraggregate com esta função personalizada:

#linear regression f-n
CalculateLinRegrDiff = function (sample){
  fit <- lm(value~ date, data = sample)
  diff(range(fit$fitted))
}

dataset2 = aggregate(value ~ id + col, dataset, CalculateLinRegrDiff(dataset))

Eu recebo o erro:

Error in get(as.character(FUN), mode = "function", envir = envir) : 
  object 'FUN' of mode 'function' was not found

O que está errado?