stdin for label: and caption: is not recognized

[lvalics]

Check this issue:
http://www.imagemagick.org/discourse-server/viewtopic.php?t=28501

Security is a compromise between being secure and usability. If you are not concerned about indirect reads (e.g. convert label:@/etc/passwd ...), remove this policy from your policy.xml configuration file:

Your command should work as expected now.

Try to add this to readme maybe?