MKVMerge Error: Demultiplexer failed to initialize

Problem

I tried to merge a subtitle SRT file with a video MP4 file using MKVMerge as explained here. It failed with this error:

$ mkvmerge -o out.mp4 foo.mp4 foo.srt 
mkvmerge v5.6.0 ('Kenya Kane') built on Jun  7 2012 15:57:56
'foo.mp4': Using the demultiplexer for the format 'QuickTime/MP4'.
Error: The demultiplexer for the file 'foo.mp4' failed to initialize:
The file could not be opened for reading, or there was not enough data to parse its headers.

Solution

Googling for this error message did not provide any solution.

I updated MKVMerge to the newer version 6.7.0. The same command on the same files worked successfully! 😊