How to `group_by` in `use amp_frequency`

[bernt-matthias]

I'm trying

data("AalborgWWTPs")
amp_frequency(AalborgWWTPs, group_by = "Plant")

but I get:

Error in apply(data$metadata[, group_by], 1, paste, collapse = " ") : 
  dim(X) must have a positive length