I am trying to write apex code (and I'm learning how) to roll up data from the on the Account from an object with a lookup relationship to the object.  

Fields from Account:  ID, DealCount__c

Fields from related object - Investment__c:  Target_Company__c (ID from Account), count of records.

when I try saving it, I'm getting the error:  Incompatible key type Object for Map<Id,Account> at line 22 column 12. (between **...** in code below)


Code:

trigger trgRollUpToAccount on Investment__c (after insert, after update) 
{
//List to store Account IDs
List<Id> AccountIDs = new List<Id>(); 
 //Get list of target accounts from investment__c
for(Investment__c i: Trigger.new) 
 {
AccountIDs.add(i.Target_Company__c);
} 
 //query to get all investment records whose account IDs are in the list
AggregateResult[] groupedResults = [Select count(id)DealCount, Target_Company__c from Investment__c 
 where  Target_Company__c in :AccountIDs Group by Target_Company__c];

 // get group of account objects to update
Map<id,Account> accountMap = new Map<id,Account>([Select id, DealCount__c from Account where id in :AccountIds]);

//iterate over grouped results and set the account map's data to match

  **for(AggregateResult ar: groupedResults) 
   { 
        
          accountMap.get(ar.get('Target_Company__c' )).DealCount__c = ar.get('DealCount');  
   }** 
    
 //Update account objects:
    try {
        update accountMap.values();
        }
        catch(DmlException e) {
        System.debug(e.getMessage());
        } 

}