Incremental not implemented for bivariate functions

On the main page (https://github.com/joehuchette/PiecewiseLinearOpt.jl/), it says that Incremental is a valid bivariate method, however I get this error when I try to use this particular bivariate method:

ERROR: LoadError: Incremental formulation for bivariate functions is not currently implemented.
Stacktrace:
[1] #piecewiselinear#84(::Symbol, ::Void, ::Function, ::JuMP.Model, ::JuMP.Variable, ::JuMP.Variable, ::PiecewiseLinearOpt.PWLFunction{2}) at /Users/a598124/.julia/v0.6/PiecewiseLinearOpt/src/jump.jl:644
[2] (::PiecewiseLinearOpt.#kw##piecewiselinear)(::Array{Any,1}, ::PiecewiseLinearOpt.#piecewiselinear, ::JuMP.Model, ::JuMP.Variable, ::JuMP.Variable, ::PiecewiseLinearOpt.PWLFunction{2}) at ./:0