buildbot exception in on asterixdb-master-docs

[buildbot]

The Buildbot has detected a build exception on builder asterixdb-master-docs 
while building . Full details are available at:
    https://ci.apache.org/builders/asterixdb-master-docs/builds/2053

Buildbot URL: https://ci.apache.org/

Buildslave for this Build: asf947_ubuntu

Build Reason: The Nightly scheduler named 'asterixDocsNightly' triggered this 
build
Build Source Stamp: [branch master] HEAD
Blamelist: 

BUILD FAILED: exception MasterShellCommand MasterShellCommand_1

Sincerely,
 -The Buildbot