[BUG] Variable not working through def

[kurtlawrence]

Current Behaviour

This works:

    open wallet-lt.csv | filter timestamp { take 10 | >= $cutoff-date }
    | let {nth 0 get:Num fiat-value} $start 
          {last get:Num fiat-value} $end
    | Table $name
    | append-row { \$end | - $start }
    | append-row { \$end | / $start | - 1 }
    | let $t
    | open executions-lt.csv | filter timestamp { take 10 | >= $cutoff-date }
    | filter real = 'true'
    | append --ret-val { let #i.buy-fiat:Num $s | get sell-fiat | - $s }
    | grp coin
    | append --sum { get value | fold 0 {:Num + $row.ret-val } }
    | sort 'sum'
    | let $cs
    | \$t
    | append-row { \ $cs | if {empty} '' { last get:Str key } }
}

However, if we abstract the last line into a definition:

def def-if-empty (t:Table d e:Expr) { \ $t | if {empty} $d $e }

def summary-col (name:Str cutoff-date:Str) {
    open wallet-lt.csv | filter timestamp { take 10 | >= $cutoff-date }
    | let {nth 0 get:Num fiat-value} $start 
          {last get:Num fiat-value} $end
    | Table $name
    | append-row { \$end | - $start }
    | append-row { \$end | / $start | - 1 }
    | let $t
    | open executions-lt.csv | filter timestamp { take 10 | >= $cutoff-date }
    | filter real = 'true'
    | append --ret-val { let #i.buy-fiat:Num $s | get sell-fiat | - $s }
    | grp coin
    | append --sum { get value | fold 0 {:Num + $row.ret-val } }
    | sort 'sum'
    | let $cs
    | \$t
    | append-row def-if-empty $cs '' { last get:Str key }
}

There is a runtime error saying that the column key is not present.

This is either a locals issue or an input issue, but the last call is not being invoked on the correct table.