I’m new to time series. While building a basic predictive model using the dowjones data set from library(fma) I ran into “> Error in xx - ff[1:n] : non-numeric argument to binary operator”.

```
library(forecast)
library(fma)
dj60 <- window(dowjones, end = 60)
dj60.rwf.drift <- rwf(dj60, h = 18, drift = TRUE)
dj60.od <- window(dowjones, start = 61)
accuracy(dj60.od, dj60.rwf.drift)
```

Returns:

Error in xx - ff[1:n] : non-numeric argument to binary operator

The preceding chain of commands works for another data set (visnights from the fpp2 library):

```
library(forecast)
library(fpp2)
vn_Melbourne_train1 <- window(visnights[, "QLDMetro"], end = c(2014, 4))
snaive_vn_Melbourne_train1 <- snaive(vn_Melbourne_train1, h = 4)
vn_Melbourne_test1 <- window(visnights[, "QLDMetro"], start = c(2015, 1), end = c(2015, 4))
accuracy(snaive_vn_Melbourne_train1, vn_Melbourne_test1)
```

I have `str()`

ed all of the objects and find them comparable.

Why does this error occur for one data set and not the other?