Can't compile ffmpeg with libnpp #628

# Lines starting with this character are ignored
--enable-avisynth
--enable-gcrypt
--enable-libmp3lame
--enable-libopus
--enable-libvorbis
--enable-libvpx
--enable-libx264
--enable-libx265
--enable-cuda
--enable-cuvid
--enable-schannel
# Zeranoe
--enable-decklink
--enable-fontconfig
--enable-frei0r
--enable-gnutls
--enable-libass
--enable-libbluray
--enable-libbs2b
--enable-libcaca
--enable-libfreetype
--enable-libfribidi
--enable-libgme
--enable-libgsm
--enable-libilbc
--enable-libmfx
--enable-libmodplug
--enable-libopencore-amrnb
--enable-libopencore-amrwb
--enable-libopenjpeg
--enable-librtmp
--enable-libsoxr
--enable-libspeex
--enable-libtheora
--enable-libtwolame
--enable-libvidstab
--enable-libvo-amrwbenc
--enable-libwavpack
--enable-libwebp
--enable-libxavs
--enable-libxvid
--enable-libzimg
#--enable-openssl
--enable-libsnappy
--enable-gpl
# Full
--enable-opencl
--enable-opengl
--enable-libcdio
--enable-libfdk-aac
--enable-libkvazaar
--enable-librubberband
--enable-libssh
--enable-libtesseract
--enable-libzvbi
--enable-chromaprint
--enable-libopenh264
--enable-libopenmpt
--enable-libnpp
--enable-libzmq
--enable-libmysofa
--enable-cuda-sdk
--enable-libflite
Except for openssl, which was causing the build to fail, everything's enabled.

Running this command

.\ffmpeg.exe -hwaccel cuvid -c:v h264_cuvid -i "4k_test.mp4" -vf scale_npp=-1:720 -c:v h264_nvenc -preset slow test.mp4

with the resulting ffmpeg build results in this error message:
[AVFilterGraph @ 000001768a61ac60] No such filter: 'scale_npp'
Error reinitializing filters!
Failed to inject frame into filter network: Invalid argument
Error while processing the decoded data for stream #0:0
Conversion failed!
Are there any additional steps I have to take in order for the accelerated scale filter to work?