I'm trying to implement lead assignment based on zip codes, and I'm running into an error with the code. I set up a Zip Code custom object, Zip_Code__c, where the name is the zipcode. Both the custom object and the Lead object have a custom text field, CA_Territory__c (this is for California zip codes only). When the zip code on a lead is entered or edited, I want to pull the CA Territory value from the custom object into the lead field. This is the code:

trigger CATerritory on Lead (before insert, before update) { 
//get the first five digits of the PostalCode from the lead 
set<string> left5zips = new set<string>(); 
for (lead l: trigger.New) if (l.postalcode !=null) left5zips.add(l.postalcode.substring(0,5));

//if no zip listed on the lead 
if(left5zips.isEmpty()) lead.ca_territory__c='None'; 

//query the zip_code object to get the zipcode (Name) and zone from the zip code object 
map<string,string> zmap=new map<string,string>(); 
  for(Zip_Code__c z :
     [Select name, ca_territory__c from Zip_Code__c WHERE name IN :left5zips])
zmap.put (z.name, z.ca_territory__c);

for(lead l:trigger.new){ 
if(zmap.containskey(l.left5zips)) 
     l.ca_territory__c=zmap.get(l.left5zips); 
} 
}
The error I am getting is 'Compile Error: Expression cannot be assigned at line -1 column -1'. This is my first attempt at a trigger, and it is based on other people's code, so I don't really know how to troubleshoot. Any ideas? Thanks!