Confusing error when trying to aggregate over an empty data-frame

[samdphillips]

For the following program I get this confusing error message that doesn't reflect what the cause is.

combine: contract violation
  expected: (and/c list? pair?)
  given: '()
  in: the rest argument of
      (->*
       ()
       #:rest
       (non-empty-listof data-frame?)
       data-frame?)
  contract from: <pkgs>/sawzall-lib/split.rkt
  blaming: <pkgs>/sawzall-lib/grouping.rkt
   (assuming the contract is correct)
  at: <pkgs>/sawzall-lib/split.rkt:11:16

#lang racket/base

(require data-frame
         sawzall
         threading)

(define df
  (let* ([df (make-data-frame)]
         [name (make-series "name" #:data (vector))]
         [val  (make-series "val" #:data (vector))])
    (df-add-series! df name)
    (df-add-series! df val)
    df))

(~> df
    (group-with "name")
    (aggregate [total (val) (for/sum ([n (in-vector val)]) n)]))