Copy / Paste doesn't work with LineEdit

[ia32]

I have a PushButton and LineEdit in a form created with Qt Designer. I wanted to copy the text of LineEdit into clipboard by clicking the PushButton, so I connected the PushButton's clicked() signal to LineEdit's copy() slot (in Signal/Slot edit mode). But it doesn't work (same with cut() slot). However, other slots like paste(), hide() etc seem to work.

Can anyone tell me what's the problem?

BTW, even if the LineEdit is read-only, I can still paste text in it by clicking the PushButton!! (when I have click() signal connected to paste() slot)