I have a trigger on a Custom Object (Ordered Ads).  The object is a Master/Detail with the Standard Object, "Account" (Account is the master). 

When inserting the Ordered Ads each day, I have a trigger that *should* associate the Ordered Ad record with the appropriate Account record based on AccountNumber.  However, it's not working.  When I attempt to enter a new record, I get an error message stating that I must enter a value in the Account__c field.  I would greatly appreciate any thoughts / advice on where I'm going wrong here...

trigger updateOrderedAdsAccount on Ordered_Ads__c (before insert) 
	
	{
        //Create record set of Account Numbers to tie to Ordered Ads
        Set<String> acctnums = new Set<String>();
        for(Ordered_Ads__c a:trigger.new)
        {
        	acctnums.add(a.Admarc_Number__c);               
        }
                
        //Create empty Maps to hold relationships. 
        MAP<String, Id> orderedAdsAcctRel = new Map<String, Id>();
       
        //Iterate through Accounts and find matches for ID based on Account Number
        for (Account u:[select id, AccountNumber from Account where External_Id__c != null AND External_Id__c in :acctnums limit 1])
        {
        	if(u.AccountNumber != null)
        	{
        		String acctnum = u.AccountNumber;
        		orderedAdsAcctRel.put(acctnum, u.id);
        	}
        	else
        	{
        		String acctnum = u.AccountNumber;
        		orderedAdsAcctRel.put(acctnum, '0014000000Ektmx');
        	}
        }
           
        //Iterate through Ordered Ads and update according to Account data from Map
        for (Ordered_Ads__c a:trigger.new)
        {
            Id u1 = orderedAdsAcctRel.get(a.Admarc_Number__c);
            a.Account__c = u1;
            
            if (a.Account__c = '')
            {
            	a.Account__c = '0014000000Ektmx';
            }
        }
    }

I do have a test case, and it passes the test case only when I select an existing account.  If I try to create a new account through the Test Case, I get the same error (Must supply a value for Account__c).

I have the trigger in my Sandbox, and it does work there.  The means for moving to Production was through the SFDC Deploy feature.

Again, I appreciate any advice on this.

Thanks.