Why can't I access customer.rules from my Threshold rules?

I have customer.rules under the default location of collection/event/common/aggregators/customer.rules and the thresholding rules under the default location of processing/event/threshold/standard_threshold.rules. All of my aggregator rules files under collection can access customer.rules, but I can't access them under processing. Both use the same line in their include files: CustomerRules,collection/event/common/aggregators/customer.rules