"Subtitle sink is blocked but we have no subtitle caps" with subparse and playsink?

[Brendan Long]

I'm trying to do a basic pipeline to test a new element, but I can't
even get it to work without my element. I just want to read subtitles
from a file and then display them on a video. Here's my launch command:

gst-launch-1.0 playsink name=sink filesrc
location=~/Downloads/sintel_en.srt ! subparse ! sink.text_sink 
videotestsrc ! video/x-raw ! sink.video_raw_sink

But I get this output:

> WARNING: from element
> /GstPipeline:pipeline0/GstPlaySink:sink/GstBin:tbin/GstSubtitleOverlay:suboverlay:
> GStreamer error: negotiation problem.
> Additional debug info:
> gstsubtitleoverlay.c(1145): _pad_blocked_cb ():
> /GstPipeline:pipeline0/GstPlaySink:sink/GstBin:tbin/GstSubtitleOverlay:suboverlay:
> Subtitle sink is blocked but we have no subtitle caps
What does this mean? Is my pipeline wrong?