Average calculation using date_diff at summary level vs. report calculations

Summary

The average date_diff calculation, used at the summary level, includes cases with a null date value, distorting the totals

Content

I've built two reports (definitions attached).  The Detail report is a report link that is called when a user drills down from the summary level.

In the Summary report, I'm using a formula to determine the average resolution time by Agent, using the date_diff function to calculate the difference between the resolution date/time and the case creation date/time.  In this Summary report, the formula is including cases that have a null resolution date/time.

In the Detailed report link, I'm using the report calculations to display an average at the bottom.  The average resolution time on the detailed report excludes cases with a null resolution date/time.