Flex dropdownlist displays [object][object]

I'm having trouble getting a drop-down list to display the values from an ArrayCollection.  The dropdown displays [object][object],[object][object] etc..  This would normally indicate that the label isn't set - but I can't see what is happening wrong.

This is the trace of my ArrayCollection:

(mx.collections::ArrayCollection)#0
  filterFunction = (null)
  length = 1
  list = (mx.collections::ArrayList)#1
    length = 1
    source = (Array)#2
      [0] (mx.collections::ArrayCollection)#3
        filterFunction = (null)
        length = 4
        list = (mx.collections::ArrayList)#4
          length = 4
          source = (Array)#5
            [0] (Object)#6
              label = "LIFE_LAPSE_MASS_SHOCK"
            [1] (Object)#7
              label = "MKT_FX_SHOCK_DN_DKK_EUR"
            [2] (Object)#8
              label = "MKT_SPD_WIDENING_SHOCK"
            [3] (Object)#9
              label = "N_LIFE_LAPSE_MASS_SHOCK"
          uid = "A68C2B85-91F0-E15F-CFF5-900BA52F1BB6"
        sort = (null)
        source = (Array)#5
    uid = "81D0DA94-1B86-6CDB-DB5D-900BA53314CE"
  sort = (null)
  source = (Array)#2

I'm getting the arraycollection from a web service :
scenAC = new ArrayCollection(ArrayUtil.toArray(event.result.Streams._WEBOUT.Value.TABLE.SCENARIOS));

and setting the dataprovider and label:

scendd.dataProvider=scenAC;
scendd.labelField='label';

<s:DropDownList  id='scendd' x="139" y="20" width="163"  enabled="false"/>

Any ideas?