Can't create a GNNGraph over degenerate graph with just a single vertex

[oxinabox]

julia> using GNNGraphs

julia> GNNGraph(([1], Int[]), graph_type=:coo)
ERROR: ArgumentError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer
Stacktrace:
  [1] _empty_reduce_error()
    @ Base ./reduce.jl:319
  [2] reduce_empty(f::Function, T::Type)
    @ Base ./reduce.jl:320
  [3] mapreduce_empty(::typeof(identity), op::Function, T::Type)
    @ Base ./reduce.jl:369
  [4] reduce_empty(op::Base.MappingRF{typeof(identity), typeof(max)}, ::Type{Int64})
    @ Base ./reduce.jl:358
  [5] reduce_empty_iter
    @ ./reduce.jl:381 [inlined]
  [6] mapreduce_empty_iter(f::Function, op::Function, itr::Vector{Int64}, ItrEltype::Base.HasEltype)
    @ Base ./reduce.jl:377
  [7] _mapreduce
    @ ./reduce.jl:429 [inlined]
  [8] _mapreduce_dim
    @ ./reducedim.jl:337 [inlined]
  [9] mapreduce
    @ ./reducedim.jl:329 [inlined]
 [10] _maximum
    @ ./reducedim.jl:987 [inlined]
 [11] _maximum
    @ ./reducedim.jl:986 [inlined]
 [12] maximum
    @ ./reducedim.jl:982 [inlined]
 [13] to_coo(coo::Tuple{Vector{Int64}, Vector{Int64}, Nothing}; dir::Symbol, num_nodes::Nothing, weighted::Bool, hetero::Bool)
    @ GNNGraphs ~/.julia/packages/GNNGraphs/qLIkY/src/convert.jl:35
 [14] to_coo
    @ ~/.julia/packages/GNNGraphs/qLIkY/src/convert.jl:29 [inlined]
 [15] GNNGraph(data::Tuple{…}; num_nodes::Nothing, graph_indicator::Nothing, graph_type::Symbol, dir::Symbol, ndata::Nothing, edata::Nothing, gdata::Nothing)
    @ GNNGraphs ~/.julia/packages/GNNGraphs/qLIkY/src/gnngraph.jl:131
 [16] GNNGraph
    @ ~/.julia/packages/GNNGraphs/qLIkY/src/gnngraph.jl:119 [inlined]
 [17] #GNNGraph#22
    @ ~/.julia/packages/GNNGraphs/qLIkY/src/gnngraph.jl:167 [inlined]
 [18] top-level scope
    @ REPL[43]:1
Some type information was truncated. Use `show(err)` to see complete types.

julia> 

[CarloLucibello]

The error message should be more informative but the constructor is supposed to be used like this:

julia> src, dest = [1,2,3], [2,3,4];

julia> GNNGraph(src, dest, num_nodes=10)
GNNGraph:
  num_nodes: 10
  num_edges: 3

julia> GNNGraph((src, dest), num_nodes=10)
GNNGraph:
  num_nodes: 10
  num_edges: 3

julia> GNNGraph((src, dest))
GNNGraph:
  num_nodes: 4
  num_edges: 3

julia> GNNGraph(num_nodes=1)
GNNGraph:
  num_nodes: 1
  num_edges: 0

In particular src and dest represent the edges and they should have equal length. If num_nodes is not passed it is inferred from the edges.

[oxinabox]

oh, I got confused when making my MWE.
My real problem was that I did have empty source and target lists,
because it was just a graph with 1 node.