Warning appearing in the log of a digest alert

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Hi there

we have a digest alert that sends message to the line managers with a list of their subordinates who have not completed a course.

This daily alert works well, but I see that sometimes in Tools/Alerts there is the warning yellow sign. This is what i see in the log

QUOTE

-----------------------Warnings---------------------------
Group by Expression Val is Null ${AlertUtils.empManager(learnerLearningRecords.assignedToId).UserName} Surrogate Id 300000187919359

UNQUOTE

This error refers to the Group by field in the Template

"${AlertUtils.empManager(learnerLearningRecords.assignedToId).UserName}"

I must say that the error is when we have a larger number of alerts sent - about 80 or more than 100. When there are a few only sent out, there is no such warning. I guess the more people it is sent to, the better chance for this warning to happen.