Basic `contourf` plot not working?

Here is a MWE:

using Pkg
Pkg.activate(".")

function peaks(x, y)
    z = 3 * (1 - x)^2 * exp(-x^2 - (y + 1)^2)
    z += -10 * (x / 5 - x^3 - y^5) * exp(-x^2 - y^2)
    z += -1/3 * exp(-(x+1)^2 - y^2)
    return z
end

x = range(-3, stop = 3, length = 100)
y = range(-2, stop = 2, length = 80)
levels = collect(-1:0.2:1)

using Plots
pyplot() # Set the backend to PyPlot
plt = plot(x, y, peaks, levels = levels)
display(plt)
savefig(plt, "test.eps")
savefig(plt, "test.png")
savefig(plt, "test.svg")
savefig(plt, "test.html")

And I get the error message:

ERROR: LoadError: MethodError: no method matching iterate(::Surface{Array{Float64,2}})
Closest candidates are:
  iterate(::Any) at essentials.jl:853
  iterate(::Any, ::Any) at essentials.jl:847
  iterate(::Core.SimpleVector) at essentials.jl:583
  ...
Stacktrace:
 [1] start(::Surface{Array{Float64,2}}) at ./essentials.jl:878
 [2] iterate(::Surface{Array{Float64,2}}) at ./essentials.jl:853
 [3] copyto!(::Array{Float64,1}, ::Surface{Array{Float64,2}}) at ./abstractarray.jl:654
 [4] _collect(::UnitRange{Int64}, ::Surface{Array{Float64,2}}, ::Base.HasEltype, ::Base.HasLength) at ./array.jl:563
 [5] collect(::Surface{Array{Float64,2}}) at ./array.jl:557
 [6] has_attribute_segments(::Plots.Series) at /Users/benoitpasquier/.julia/packages/Plots/ex9On/src/utils.jl:691
 [7] iter_segments(::Plots.Series) at /Users/benoitpasquier/.julia/packages/Plots/ex9On/src/utils.jl:197
 [8] py_add_series(::Plots.Plot{Plots.PyPlotBackend}, ::Plots.Series) at /Users/benoitpasquier/.julia/packages/Plots/ex9On/src/backends/pyplot.jl:469
 [9] _before_layout_calcs(::Plots.Plot{Plots.PyPlotBackend}) at /Users/benoitpasquier/.julia/packages/Plots/ex9On/src/backends/pyplot.jl:945
 [10] prepare_output(::Plots.Plot{Plots.PyPlotBackend}) at /Users/benoitpasquier/.julia/packages/Plots/ex9On/src/plot.jl:254
 [11] display at /Users/benoitpasquier/.julia/packages/Plots/ex9On/src/output.jl:144 [inlined]
 [12] gui at /Users/benoitpasquier/.julia/packages/Plots/ex9On/src/output.jl:134 [inlined]
 [13] display(::REPL.REPLDisplay{REPL.LineEditREPL}, ::MIME{Symbol("text/plain")}, ::Plots.Plot{Plots.PyPlotBackend}) at /Users/benoitpasquier/.julia/packages/Plots/ex9On/src/output.jl:150
 [14] display(::REPL.REPLDisplay, ::Any) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v0.7/REPL/src/REPL.jl:135
 [15] display(::Plots.Plot{Plots.PyPlotBackend}) at ./multimedia.jl:287
 [16] top-level scope at none:0
 [17] include at ./boot.jl:317 [inlined]
 [18] include_relative(::Module, ::String) at ./loading.jl:1038
 [19] include(::Module, ::String) at ./sysimg.jl:29
 [20] include(::String) at ./client.jl:398
 [21] top-level scope at none:0
in expression starting at /Users/benoitpasquier/Projects/JuliaPlotting/plot_a_contour_of_peaks.jl:18

where plot_a_contour_of_peaks.jl:18 is the display(plt) line in the MWE above. Am I doing something wrong?

JuliaPy / PyPlot.jl

Basic `contourf` plot not working? #390