When attempting to create my Apex trigger for this Trailhead development exercise, I receive an error. I don't understand how this is possible, because the records haven't been updated, and still aren't updating.
"Challenge not yet complete... here's what's wrong:
Setting 'Match_Billing_Address__c' to false updated the records anyway. The trigger should only act when Match_Billing_Address__c is true."

Here's my code:
trigger AccountAddressTrigger on Account (before insert, before update) {

    for(Account a : Trigger.new){
        If (a.Match_Billing_Address__c = true) {
            a.ShippingPostalCode = a.BillingPostalCode;
        }   
    }

}