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After updating from 2.x-dev (2011-Jun-29) to 2.0 alpha the attached error appears.
The current dev (2.x-dev (2012-Jan-12)) version dose as well not work.
To solve this issue I have added a default case at the switch statement:
default: $default_value = $element['#values']['average'];
(code is added a line 637)
But I think this is only a workaround ....