(Bloomberg) -- State Street said it incorrectly invoiced at least $200 million in asset servicing expenses to clients over a period of 18 years and will compensate customers.

“Annual amounts invoiced for these expenses ranged from approximately $9 million in the early years to approximately $36 million in 2014.,” the Boston-based firm said today in a statement.

“The actual amount to be reimbursed to clients will not be known until the review is completed, and that amount could differ materially from the Company’s preliminary assessment.”

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