diff --git a/bin/muxsa-pngaac2mp4 b/bin/muxsa-pngaac2mp4
index 587eed7..6eb470d 100755
--- a/bin/muxsa-pngaac2mp4
+++ b/bin/muxsa-pngaac2mp4
@@ -1,6 +1,6 @@
 #!/bin/bash
 
-# muxsa-pngaac2mp4 
+# muxsa-pngaac2mp4
 #
 # part of muxsa, https://git-nks-public.tik.uni-stuttgart.de/edu/muxsa
 #
@@ -8,23 +8,23 @@
 # a series of slides in png file format (created with muxsa-kvm2png),
 # a soundtrack.aac (recorded with audacity), and
 # a ffmpeg concat multiplexer sequence file (created with muxsa-al2fc)
-# and multiplex everything together 
+# and multiplex everything together
 # into a mp4 video of the slideshow with background narration
 
 # MIT License
-# 
+#
 # Copyright (c) 2023 Sebastian Kiesel <sebastian.kiesel@tik.uni-stuttgart.de>
-# 
+#
 # Permission is hereby granted, free of charge, to any person obtaining a
 # copy of this software and associated documentation files (the "Software"),
 # to deal in the Software without restriction, including without limitation
 # the rights to use, copy, modify, merge, publish, distribute, sublicense,
 # and/or sell copies of the Software, and to permit persons to whom the
 # Software is furnished to do so, subject to the following conditions:
-# 
+#
 # The above copyright notice and this permission notice shall be included
 # in all copies or substantial portions of the Software.
-# 
+#
 # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 # IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 # FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
@@ -36,7 +36,7 @@
 ############################################################################
 # see https://trac.ffmpeg.org/wiki/Slideshow
 
-echo "This is muxsa-pngaac2mp4." 
+echo "This is muxsa-pngaac2mp4."
 
 # first, we need to define 3 file names - check command line, then defaults:
 # SLIDES_IN - concat multiplexer control file with png slide names
@@ -74,6 +74,10 @@ shift "$((OPTIND-1))"
 if [ -z "$VIDEO_OUT" -a -n "$1" ] ; then
     VIDEO_OUT="$1"
 fi
+# last resort: default output file name
+if [ -z "$VIDEO_OUT" ] ; then
+    VIDEO_OUT="out.mp4"
+fi
 
 if [ -z "$AUDIO_IN" -a -r "soundtrack.m4a" ] ; then
     echo "Setting AUDIO_IN to default file name soundtrack.m4a"