When I try to initialize the model of ComboBox, weird error pop out

Qt Code:
  1. import QtQuick 2.2
  2. import QtQuick.Controls 1.1
  3.  
  4. Rectangle {
  5. width: 100
  6. height: 62
  7.  
  8. ListModel{
  9. id: modelA
  10. }
  11.  
  12. ComboBox{
  13. model: modelA
  14. }
  15.  
  16. Component.onCompleted: {
  17. modelA.append({"source" : "hhhh"})
  18. }
  19. }
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error message

file:///C:/Qt/Qt5.2.0/5.2.0/mingw48_32/qml/QtQuick/Controls/ComboBox.qml:496: TypeError: Cannot read property 'constructor' of undefined

How could I fix this error?