# Error building FFTW
**URL:** https://discourse.julialang.org/t/error-building-fftw/26827
**Category:** General Usage
**Created:** [July 26, 2019, 6:41am UTC](https://discourse.julialang.org/t/error-building-fftw/26827 "2019-07-26T06:41:09Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@IljaK91](https://discourse.julialang.org/u/IljaK91)
#### Post date: [July 26, 2019, 6:41am UTC](https://discourse.julialang.org/t/error-building-fftw/26827/1 "2019-07-26T06:41:09Z")
Does anybody understand what the problem is here and how I can fix it?
```julia
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
``
```
---
### Author:  [@IljaK91](https://discourse.julialang.org/u/IljaK91)
#### Post date: [July 26, 2019, 7:20am UTC](https://discourse.julialang.org/t/error-building-fftw/26827/2 "2019-07-26T07:20:50Z")
This solved the problem:
```julia
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\").")
```