Unable to build CentOS 6 on CentOS 7 Smart Proxy

We receive a series of error messages relating to the kickstart file. An example:
The following problem occurred on line 38 of the kickstart file:
no such option: --vckeymap
install exited abnormally [1/1]

This is similar to the errors seen when using ksvalidate on a kickstart file for another OS version.
We need to build several CentOS 6 systems for a migration. Has anyone else experienced this issue?

Expected outcome:
A completed build. This system builds CentOS 7 VMs with no issues. CentOS 6 is set up identically and by the book.

Foreman and Proxy versions:
Foreman 1.23.0
Katello 3.13.1
Both from RPM.

Foreman and Proxy plugin versions:

Distribution and version:
CentOS 7

Other relevant data:

Last line in anaconda.log is “INFO anaconda: Running kickstart %%post script(s)”. Nothing appears after that.