Reports :: Average Of Time In Minutes And Seconds

Basically, I get daily reports from the client and the AHT and ACW values come in seconds. I've recently added code to the query changing the data to nn:ss which works perfectly. Below is some info:

tblTelephony
[AgentName]
[Calls] (number of calls answered)
[AHT] (in seconds)
[ACW] (in seconds)

[Code] ....

The above code works very well, where I'm stuck is in the attempt to average the fields [AHTMinSec] & [ACWMinSec] by Agent in a report.

Using [AHTMinSec] as an example:

I have the daily values for each agent [AHTMinSec] in the detail, Avg([AHTMinSec]) in the Agent group footer showing each agent's average, Avg([AHTMinSec]) in the Team group footer showing team averages and Avg([AHTMinSec]) in the report footer showing the campaign average including all teams. When I run the report the details are hidden providing each agent's average, the team average, and the campaign average.

The report worked just fine until I converted to minutes with the above code. Is there a reason that I'm getting an error stating that the calculation is too complex? I've done enough research to determine that the db can't Sum in order to average...