Help with SUM expression

I’m running into an issue with using SUM…

Specifically, I’m attempting to sum up the total strength of a certain class of units, where the strength of each unit is expressed by the layer level.

Sum(Strength_Level,"{Class==“Infantry”}",“Group A”)

Any ideas why this may not be working?

Figured it out, the property wants " " around it…

So the below works:
Sum(“Strength_Level”,"{Class==“Infantry”}",“Group A”)