Hi,
i have a Trigger where I am searching for a value in a case to lookup on accounts to fill in the account id to the case.

The Trigger works well so far in the sandbox, but when i want to validate to get in in production deployed, it returns the following issue:

System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, AccountOnCases: execution of BeforeInsert caused by: System.QueryException: Non-selective query against large object type (more than 100000 rows). Consider an indexed filter or contact salesforce.com about custom indexing. Even if a field is indexed a filter might still not be selective when: 1. The filter value includes null (for instance binding with a list that contains null) 2. Data skew exists whereby the number of matching rows is very large (for instance, filtering for a particular foreign key value that occurs many times) Trigger.AccountOnCases: line 7, column 1 


Here is the trigger:

trigger AccountOnCases on Case (before insert) {
List<String> Account = new List<String>(); 
for (Case a:Trigger.new)
{
Account.add(a.Customer_Number_Lookup_C__c);
}
List <Account> AccountList = [Select ID from Account where Customer_Number__c in :Account];
    if (AccountList.size() > 0) {
        for (Integer i = 0; i < Trigger.new.size(); i++)
            
        
if (Trigger.new[i].Customer_Number_C__c != null) 
{
Trigger.new[i].AccountID = AccountList[i].ID;
}
else
{
Trigger.new[i].AccountID = null;
}
}
}

Do you have an idea, how to solve this?

Thanks in advance

Ralf