Cannot add new errata using spacecmd softwarechannel_adderrata, reports "No errata to add"

Issue

When trying to add errata to a cloned channel using spacecmd, an error is returned indicating there's no matching errata:

spacecmd {SSM:0}> softwarechannel_adderrata rhel-x86_64-server-5 rhel-x86_64-server-54 RHSA-2012:1047
WARNING: No errata to add