title: Unable to find a suitable output format for ' : Invalid argument
body: <p><code>ffmpeg -i a.webm -i b.webm -filter_complex '[0:v]pad=iw*2:ih[int];[int][1:v]overlay=W/2:0[vid]' -map [vid] -c:v libx264 -crf 23 -preset veryfast outputfin.webm</code></p>
  <p>This was meant to be a simple code to merge two videos side-by-side. However, it is throwing the following error:</p>
  <pre><code>'NULL @ 0x55aafb8828e0] Unable to find a suitable output format for '
  : Invalid argument
  </code></pre>
  <p>What is the error here?</p>