Object doesn't support property or method 'filter'

I'm running into an IE8 issue which I initially thought was due to lack of IE8 support for indexOf() property but that didn't seem to be the case. I implemented the indexOf() through prototype but still keep getting the error messsage -

Object doesn't support property or method 'filter'

My code is and it errors out on the passedArray.filter line.

  function consolidatedFilters(passedArray, passedFilter, passedFilterType)
    if (passedFilterType == "specialty")
    {       
        var filteredArray = passedArray.filter(
            function(el)
            {
               for (i in passedFilter) 
               {
                    if (passedFilter[i] == el[8]) 
                    {
                        return true;
                    }
               }
               return false
             }
        );      
        return filteredArray;
    }