FFMPEG image overlaying video: crash

I have a simple script that overlays a set of images on top of a video:

ffmpeg -y -i "basevideo.mov" -loop 1 -i "animation/middle_%2d.png" -filter_complex \
"[1:v]setpts=1.0*PTS[midAnimation]; \
[0:v][midAnimation]overlay=shortest=1[outvideo]" \
-map [outvideo] "out.mov"

When the script gets to a certain point, I get an "Error while filtering" message (in bright red), and then it hangs on the last frame (I need to "ctrl-c" a few times to exit).

This doesn't happen on all videos, but enough of them to be annoying.

My environment:

ffmpeg version 3.3.3 Copyright (c) 2000-2017 the FFmpeg developers
  built with Apple LLVM version 8.1.0 (clang-802.0.42)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/3.3.3 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-libmp3lame --enable-libx264 --enable-libxvid --enable-opencl --disable-lzma --enable-vda
  libavutil      55. 58.100 / 55. 58.100
  libavcodec     57. 89.100 / 57. 89.100
  libavformat    57. 71.100 / 57. 71.100
  libavdevice    57.  6.100 / 57.  6.100
  libavfilter     6. 82.100 /  6. 82.100
  libavresample   3.  5.  0 /  3.  5.  0
  libswscale      4.  6.100 /  4.  6.100
  libswresample   2.  7.100 /  2.  7.100
  libpostproc    54.  5.100 / 54.  5.100