Error building FFTW

General Usage
1

Does anybody understand what the problem is here and how I can fix it?

PS C:\Users\Ilja> jlpkg build FFTW
  Building Conda → `C:\Users\Ilja\.julia\packages\Conda\kLXeC\deps\build.log`
  Building FFTW ─→ `C:\Users\Ilja\.julia\packages\FFTW\2okGQ\deps\build.log`
┌ Error: Error building `FFTW`:
│ ERROR: LoadError: KeyError: key "JULIA_FFTW_PROVIDER" not found
│ Stacktrace:
│  [1] (::getfield(Base, Symbol("##457#458")))(::String) at .\env.jl:79
│  [2] access_env(::getfield(Base, Symbol("##457#458")), ::String) at .\env.jl:14
│  [3] getindex(::Base.EnvDict, ::String) at env.jl:79
│  [4] top-level scope at C:\Users\Ilja\.julia\packages\FFTW\2okGQ\deps\build.jl:12
│  [5] include at .\boot.jl:328 [inlined]
│  [6] include_relative(::Module, ::String) at .\loading.jl:1094
│  [7] include(::Module, ::String) at .\Base.jl:31
│  [8] include(::String) at client.jl:431
│  [9] top-level scope at none:5
│ in expression starting at C:\Users\Ilja\.julia\packages\FFTW\2okGQ\deps\build.jl:11
└ @ Pkg.Operations C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.2\Pkg\src\backwards_compatible_isolation.jl:647
``
2

This solved the problem:

    error("Unrecognized JULIA_FFTW_PROVIDER \"$provider\".\n",
          "To fix this, set ENV[\"JULIA_FFTW_PROVIDER\"] to \"FFTW\" or \"MKL\"\n",
          "and rerun Pkg.build(\"FFTW\").")