Fraud examination, chapter 4, case #9

Robert was the chief teller in a large New York bank. Over a period of three years, he embezzled $1.5 million. He took the money by manipulating dormant accounts. Unfortunately, Robert was both responsible for handling dormant accounts and for dealing with complaints from customers. When a customer would complain about his account, Robert was always the one to explain the discrepancy. He usually used the excuse that it's a computer error. What internal control weakness allowed this fraud to occur?