Preparing to enter a Costco store in Alhambra, California. 
Preparing to enter a Costco store in Alhambra, California.  - 
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American Express CEO Ken Chennault held an unscheduled call with analysts today to warn everybody that AmEx's credit-card partnership with warehouse retailer Costco is going to expire next year.

AmEx shares lost 6.5 percent as a result.

Most interesting factoid of the whole story?

The average Costco customer has an income of $100,000 a year.

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Follow Kai Ryssdal at @kairyssdal