diff --git a/src/player/Thread.cxx b/src/player/Thread.cxx
index 6e57ecb4a..01cca7b8f 100644
--- a/src/player/Thread.cxx
+++ b/src/player/Thread.cxx
@@ -367,6 +367,7 @@ Player::StartDecoder(std::unique_lock<Mutex> &lock,
 		     std::shared_ptr<MusicPipe> _pipe,
 		     bool initial_seek_essential) noexcept
 {
+	assert(!decoder_starting);
 	assert(queued || pc.command == PlayerCommand::SEEK);
 	assert(pc.next_song != nullptr);