Error for unstacking an empty dataframe

[kollch]

When attempting to run the unstack function on an empty dataframe, the error produced references an internal MethodError:

ERROR: no method matching DataFrame(::Vector{Vector{Any}}, ::Vector{Any}; copycols=false)

How to replicate:

df = DataFrame(a = [], b = [], c = [])
unstack(df, :b, :c)

I think it'd be nice to have a check for this, especially considering stack doesn't appear to have a similar problem.

[bkamins]

Thank you for reporting. This is a problem with broadcasting in base. Fixed in #2842