[SalesForce] AuraEnabled Apex method wont return any callback response

I have a dropdown in my lightning component that calls an apex method on the backend. There are two options in this dropdown. For one, everything works as is expected. For the other option, I have stepped through the controller, helper, and apex class and everything is as expected until I return the object to my callback function. At this point, absolutely nothing is returned to the callback function.

Here is my js controller:

getAccountSummary: function(component, selectedLocation){
    var action = component.get("c.getAccountSummaryApex");
    action.setParams({
        "selectedLocation": selectedLocation,
    });
    action.setCallback(this, function(response) {
        alert('response = ' + response);
        var state = response.getState();
        if (component.isValid() && state == "SUCCESS") {
            var result = response.getReturnValue();
            component.set("v.accountSummary", result);
        }
    });       
    // Send action off to be executed
    alert('called apex');
    $A.enqueueAction(action);
},   

Here is my apex method:

 @AuraEnabled
 public static AccountSummary getAccountSummaryApex(String selectedLocation{
   User currentUser = getCurrentUser();
   List<Account> relatedAccounts = getRelatedAccount(currentUser, null);
   List<Zuora__CustomerAccount__c> billingAccounts = 
     getBillingAccounts(relatedAccounts, false);

   List<Zuora__ZInvoice__c> totalInvoiceList = new List<Zuora__ZInvoice__c>();

   totalInvoiceList = [SELECT id, Name, Zuora__Zuora_Id__c, Zuora__BillingAccount__r.Zuora__Zuora_Id__c, Zuora__Account__c, 
                         Zuora__Account__r.Payment_Method__c, Zuora__Account__r.Location2__c, 
                         Zuora__Account__r.Location2__r.Name, Payment_Status__c, Zuora__Payment_Term__c, Zuora__DueDate__c,
                         Zuora__Balance2__c, Outstanding_amount_before_credit__c, Zuora__CreditBalanceAdjustmentAmount__c, 
                         Zuora__Description__c, Zuora__TotalAmount__c
                         FROM Zuora__ZInvoice__c
                         WHERE Zuora__Account__c IN :relatedAccounts AND Zuora__Account__r.Location2__c =: selectedLocation 
                         AND Payment_Status__c != :paymentStatusProcessing AND Zuora__Balance2__c > 0];  

    AccountSummary acctSum = new AccountSummary(); 
    if(!totalInvoiceList.isEmpty()) 
    {
        for (Zuora__ZInvoice__c invoice:totalInvoiceList) {
            if (invoice.Zuora__DueDate__c < Date.today() && invoice.Outstanding_amount_before_credit__c != null && 
                    invoice.Zuora__CreditBalanceAdjustmentAmount__c != null && invoice.Zuora__Balance2__c != null) {

                acctSum.previousBalance = acctSum.previousBalance + invoice.Outstanding_amount_before_credit__c;
                acctSum.creditApplied = acctSum.creditApplied + invoice.Zuora__CreditBalanceAdjustmentAmount__c;
                acctSum.totalBalanceDue = acctSum.totalBalanceDue + invoice.Zuora__Balance2__c;

            } else if (invoice.Zuora__DueDate__c >= Date.today() && invoice.Outstanding_amount_before_credit__c != null && 
                           invoice.Zuora__CreditBalanceAdjustmentAmount__c != null && invoice.Zuora__Balance2__c != null){

                acctSum.newCharge = acctSum.newCharge + invoice.Outstanding_amount_before_credit__c;
                acctSum.creditApplied = acctSum.creditApplied + invoice.Zuora__CreditBalanceAdjustmentAmount__c;
                acctSum.totalBalanceDue = acctSum.totalBalanceDue + invoice.Zuora__Balance2__c;
            }
        }
    }
    system.debug('return accountSummary = ' + acctSum);
    return acctSum;           
}   

The system.debug contains the expected value for either option. Is there any reason why 'acctSum' wouldnt get returned correctly after the value is correct on the line before.

Also, acctSum is of type AccountSummary, which can be seen below.

public class AccountSummary {

    public AccountSummary(){
        this.previousBalance = 0.0;
        this.newCharge = 0.0;
        this.creditApplied = 0.0;
        this.totalBalanceDue = 0.0;
    }

    @AuraEnabled 
    public double previousBalance{get;set;}
    @AuraEnabled 
    public double newCharge{get;set;}
    @AuraEnabled 
    public double creditApplied{get;set;}
    @AuraEnabled 
    public double totalBalanceDue{get;set;}

}