Group as variable in statsplots

Hi-

I would like to pass a variable to the keyword group in StatsPlots, but I receive an error. Here is a simple example:

using DataFrames,StatsPlots
df = DataFrame(a=rand(100),b=rand(100),c=rand(1:2,100),d=rand(1:2,100))
group = (:c,:d)
for c in (:a,:b)
    p = @df df histogram(cols(c),group=cols(group),layout=(2,2))
    display(p)
end

Error:


MethodError: no method matching extractGroupArgs(::Array{Int64,2}, ::Array{Float64,1})
Closest candidates are:
  extractGroupArgs(!Matched::AbstractArray{T,1} where T, ::Any...; legendEntry) at /home/dfish/.julia/packages/Plots/oiirH/src/args.jl:1078
  extractGroupArgs(!Matched::Tuple, ::Any...) at /home/dfish/.julia/packages/Plots/oiirH/src/args.jl:1091
  extractGroupArgs(!Matched::NamedTuple, ::Any...) at /home/dfish/.julia/packages/Plots/oiirH/src/args.jl:1101

I also triedgroup=group and group = cols.(group) without success.