Two Democratic Senate Banking Committee members have asked Walmart, Green Dot and MasterCard to explain an outage in which Walmart MoneyCard users were blocked from accessing their funds.

In May, Walmart-branded prepaid debit card users experienced a service interruption due to a slowdown in transaction processing by MasterCard, Green Dot's payment processing partner.

In a joint letter dated June 28, U.S. Sens. Sherrod Brown, D-Ohio, and Bob Menendez, D-N.J., requested that the companies' chief executives provide a timeline of the interruptions with explanations detailing how the problem happened, how many Ohio and New Jersey customers were affected, reimbursement offers for harmed consumers and what the companies are doing to prevent similar disruptions.

The letter also requested the companies distinguish the circumstances of the May glitch from problems that arose at the prepaid card firm RushCard last year, when 440,000 customers claimed they had been frozen out of their accounts for more than two weeks after a technical breakdown. This May, RushCard agreed to a preliminary settlement of $20.5 million to resolve the issue.

"These service interruptions are a clear indication that more oversight and consumer protection is needed in the prepaid card market," Brown and Menendez wrote, as they commended the Consumer Financial Protection Bureau for increasing oversight of the prepaid card market. "We are encouraged that the CFPB proposed rules to regulate prepaid cards, in order to ensure fair dealing in this market."

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