diff --git a/NEWS b/NEWS
index 18e8a0c28..40df6aed9 100644
--- a/NEWS
+++ b/NEWS
@@ -1,4 +1,6 @@
 ver 0.18.21 (not yet released)
+* decoder
+  - ffmpeg: fix time stamp underflow
 
 ver 0.18.20 (2014/12/08)
 * decoder
diff --git a/src/decoder/FfmpegDecoderPlugin.cxx b/src/decoder/FfmpegDecoderPlugin.cxx
index 8a0937903..9a00bf3c4 100644
--- a/src/decoder/FfmpegDecoderPlugin.cxx
+++ b/src/decoder/FfmpegDecoderPlugin.cxx
@@ -284,10 +284,13 @@ ffmpeg_send_packet(Decoder &decoder, InputStream &is,
 		   AVFrame *frame,
 		   uint8_t **buffer, int *buffer_size)
 {
-	if (packet->pts >= 0 && packet->pts != (int64_t)AV_NOPTS_VALUE)
-		decoder_timestamp(decoder,
-				  time_from_ffmpeg(packet->pts - start_time_fallback(*stream),
-				  stream->time_base));
+	if (packet->pts >= 0 && packet->pts != (int64_t)AV_NOPTS_VALUE) {
+		auto start = start_time_fallback(*stream);
+		if (packet->pts >= start)
+			decoder_timestamp(decoder,
+					  time_from_ffmpeg(packet->pts - start,
+							   stream->time_base));
+	}
 
 	AVPacket packet2 = *packet;