Why does `substitute` work in multiple lines, but not in a single line?

I was attempting to answer this nice question about creating a non-standard evaluating function for a data.table object, doing a grouped sum. Akrun came up with a lovely answer which I'll simplify here:

akrun <- function(data, var, group){
 var <- substitute(var)
 group <- substitute(group)
 data[, sum(eval(var)), by = group]
}

library(data.table)
mt = as.data.table(mtcars)
akrun(mt, cyl, mpg)
#    group    V1
# 1:     6 138.2
# 2:     4 293.3
# 3:     8 211.4

I was also working on an answer, and had close to the same answer, but with the substitutes inline with the rest. Mine results in an error:

gregor = function(data, var, group) {
  data[, sum(eval(substitute(var))), by = substitute(group)]
} 

gregor(mt, mpg, cyl)
# Error in `[.data.table`(data, , sum(eval(substitute(var))), by = substitute(group)) : 
#  'by' or 'keyby' must evaluate to vector or list of vectors 
#  (where 'list' includes data.table and data.frame which are lists, too) 

At its face, my function is a simple substitution of Akrun's. Why doesn't it work?

Note that both substitutions cause problems, as shown here:

gregor_1 = function(data, var, group) {
  var = substitute(var)
  data[,sum(eval(var)), 
       by = substitute(group)]
} 
gregor_1(mt, mpg, cyl)
# Same error as above


gregor_2 = function(data, var, group) {
  group = substitute(group)
  data[,sum(eval(substitute(var))), 
       by = group]
} 
gregor_2(mt, mpg, cyl)
# Error in eval(substitute(var)) : object 'mpg' not found