Error in if (stderr < 10 * .Machine$double.eps * max(abs(mx), abs(my)))
            stop("data are essentiallyconstant") :
missing value where TRUE/FALSE needed
In addition: Warning messages:
1: In mean.default(x) : argument is not numeric or logical: returning NA
2: In mean.default(y) : argument is not numeric or logical: returning NA

x = as.factor(1:5)
y = as.factor(1:5)

t.test(x, y)

Error in if (stderr < 10 * .Machine$double.eps * max(abs(mx), abs(my))) stop("data are essentially constant") :
  missing value where TRUE/FALSE needed
In addition: Warning messages:
1: In mean.default(x) : argument is not numeric or logical: returning NA
2: In mean.default(y) : argument is not numeric or logical: returning NA

x + y

[1] NA NA NA NA NA
Warning message:
In Ops.factor(x, y) : + not meaningful for factors