How to Clear the contend of a Muti Select field type with a URL Formula

I have a formula URL button and I'm trying to clear a Multi select field.

I always just use this type of syntax to clear either numeric, text, date, user et=c  fields.  But when I try to clear the field I get the API error message that I must supply a valid choice for field ID 357.

Is there some trick to clearing a multi select field type with a formula URL button?  Here is a portion of my button formula.

var text ClearSelectedPendings = URLRoot() & "db/" & Dbid() & "?act=API_EditRecord&rid=" & [Record ID#]
& "&_fid_357=";

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