merge data frames "not a slot in class data.frame" - r

Question

I use the book "A practical guide to geostatistical mapping" from T. Hengl, which also offers the code to reproduce the results. Unfortunately, loads of the code contained is deprecated or even defunct. I was able to restore most of the code, but now I'm stuck with something seemingly simple: merging two data frames. My error:
Error in (function (cl, name, valueClass) : ‘data’ is not a slot in class “data.frame”
Here the code to reproduce that error:
library(gstat)
library(rgdal)
library(sp)
# load the data:
data(meuse)
coordinates(meuse) <- ~x+y
proj4string(meuse) <- CRS("+init=epsg:28992")
download.file("http://spatial-analyst.net/book/system/files/meuse.zip", destfile=paste(getwd(), "meuse.zip", sep="/"))
grid.list <- c("ahn.asc", "dist.asc", "ffreq.asc", "soil.asc")
# unzip the maps in a loop:
for(j in grid.list){
fname <- unzip("meuse.zip", file=j)
print(fname)
file.copy(fname, paste("./", j, sep=""), overwrite=FALSE)
}
# load grids to R:
meuse.grid <- readGDAL(grid.list[1])
# fix the layer name:
names(meuse.grid)[1] <- sub(".asc", "", grid.list[1])
for(i in grid.list[-1]) {
meuse.grid#data[sub(".asc", "", i[1])] <- readGDAL(paste(i))$band1
}
names(meuse.grid)
proj4string(meuse.grid) <- CRS("+init=epsg:28992")
meuse.ov <- over(meuse, meuse.grid)
str(meuse.ov)
meuse.data <- meuse[c("zinc", "lime")]#data
str(meuse.data)
meuse.ov#data <- merge(meuse.ov, meuse.data)
This is really confusing, as both data frames (meuse.ov and meuse.data) seem identical in their structure:
> str(meuse.ov)
'data.frame': 155 obs. of 4 variables:
$ ahn : int 3214 3402 3277 3563 3406 3355 3428 3476 3522 3525 ...
$ dist : num 0.00136 0.01222 0.10303 0.19009 0.27709 ...
$ ffreq: int 1 1 1 1 1 1 1 1 1 1 ...
$ soil : int 1 1 1 2 2 2 2 1 1 2 ...
and
> str(meuse.data)
'data.frame': 155 obs. of 2 variables:
$ zinc: num 1022 1141 640 257 269 ...
$ lime: Factor w/ 2 levels "0","1": 2 2 2 1 1 1 1 1 1 1 ...
I tried resolving this with looking things up on stackoverflow, but nothing did work. The (not working) legacy code in the book suggested this (for your understanding maybe):
meuse.ov <- overlay(meuse.grid, meuse)
meuse.ov#data <- cbind(meuse.ov#data, meuse[c("zinc", "lime")]#data)