StandardController of Account to show a pageblocktable of related child object from an extension errors out

I have a object Competitor_Mapping__c that has lookup with Account. In the page, i have used standard controller of Account to fetch the current Account id from URL, used in a SOQL to populate a list of associated Competitor_Mapping__c records, within an extension, which errors out Error: Invalid field Competitor_Product__c for SObject Account Competitor_Product__c is a field of Competitor_Mapping__c.