diff --git a/ffmpeg-myth.scm b/ffmpeg-myth.scm
index f4fb661..59aed1b 100755
--- a/ffmpeg-myth.scm
+++ b/ffmpeg-myth.scm
@@ -370,6 +370,8 @@
 	    ;; for some reason, ffmpeg's i-frame # is 2 less than that mythtv reports
 	    ;; after frame 0
 	    ;; (i-frame offset time)
+	    ((not (number? (first (car l))))
+		 (type-9/33-marks (cdr l) acc))
 	    ((null? acc)
 	     (type-9/33-marks (cdr l) (list (list (fourth (car l)) (second (car l)) (inexact->exact (round (* (- (first (car l)) start_pts) 1000)))))))
 	    (#t