Can't seek with --start if video has only single keyframe

[Kagami]

Test video

http://dump.bitcheese.net/files/ocaxeto/aila.webm

Test output

$ mpv --no-config --start 1 --pause aila.webm 
Playing: aila.webm
 (+) Video --vid=1 (*) (f) (vp9)
 (+) Audio --aid=1 --alang=jpn (vorbis)
[mkv] Discarding potentially broken or useless index.
[mkv] no target for seek found
[mkv] Corrupt file detected. Trying to resync starting from position 50731...
Error initializing audio.
Audio: no audio


Exiting... (Errors when loading file)
$ mpv --version
mpv git-0e0f07b (C) 2000-2015 mpv/MPlayer/mplayer2 projects
 built on UNKNOWN
ffmpeg library versions:
   libavutil       55.5.100
   libavcodec      57.14.100
   libavformat     57.14.100
   libswscale      4.0.100
   libavfilter     6.14.101
   libswresample   2.0.100
ffmpeg version: N-76461-gfd0bf45

Key frames

$ ffprobe -loglevel quiet -show_frames -select_streams v aila.webm | grep -c key_frame=1
1