Apply WidgetAdapter to CheckboxSelectMultiple.
Apply WidgetAdapter to CheckboxSelectMultiple.

Without this, any project that uses django.forms.widgets.CheckboxSelectMultiple crashes needlessly in any step that touches the database, because of Telepath's "don't know how to pack object" error.

Simply applying WidgetAdapter to CheckboxSelectMultiple seems to work fine, though I haven't tested it in any robust way. I just went in to my site that uses CheckboxSelectMultiple in several places, and confirmed that it still works.
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