We have a matrix parameterized job with a single string parameter 'ChildJobs' which was designed to get the comma separated list of downstream job names to execute.

As a build step we are using the Parameterized Trigger Plugin builder with its 'Projects to Build' parameter set to ${ChildJobs}.

When a single job name is specified with 'ChildJobs' - all works well. When multiple, comma separated names are specified the following error is generated and the matrix job fails.

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Started by upstream project "TestChildJobs" build number 6
originally caused by:
Started by user anonymous
[EnvInject] - Loading node environment variables.
Building on master in workspace /jenkins-server/Home/jobs/TestChildJobs/workspace/498111b4
ERROR: Build aborted. Can't trigger undefined projects. -1 of the below project(s) can't be resolved:
> ${ChildJobs}
Check your configuration!
Notifying upstream projects of job completion
Finished: FAILURE
----------------

One note though: all the specified downstream jobs are actually triggered and executed as needed. Only the matrix job's status is set to failure and no other build steps are executed. Not sure if the fact that the job defined as matrix has any impact here.