Message box comes up with wrong icon and no default button

[PierreA]

bool TinCanvas::mouseCheckImported()
{
  bool ret;
  if (doc.pl[plnum].whichBreak0Valid==1)
  {
    QMessageBox msgBox(this);
    msgBox.setText(tr("You have imported breaklines."));
    msgBox.setInformativeText(tr("Do you want to edit?"));
    msgBox.setStandardButtons(QMessageBox::Yes|QMessageBox::No);
    msgBox.setIcon(QMessageBox::Question);
    msgBox.setDefaultButton(QMessageBox::No);
    ret=msgBox.exec()==QMessageBox::Yes;
  }
  else
    ret=true;
  return ret;
}

bool TinCanvas::mouseCheckImported()
{
  bool ret;
  if (doc.pl[plnum].whichBreak0Valid==1)
  {
    QMessageBox msgBox(this);
    msgBox.setText(tr("You have imported breaklines."));
    msgBox.setInformativeText(tr("Do you want to edit?"));
    msgBox.setStandardButtons(QMessageBox::Yes|QMessageBox::No);
    msgBox.setIcon(QMessageBox::Question);
    msgBox.setDefaultButton(QMessageBox::No);
    ret=msgBox.exec()==QMessageBox::Yes;
  }
  else
    ret=true;
  return ret;
}

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When I run the program and click on an edge after importing breaklines, this message box pops up, but its icon is (i), not (?), and neither button looks like the default. I've tested it and found that the default button is, in fact, "No".

I'm running Kubuntu Xenial; "ldd `which kate`" shows that it is using Qt5, as is my program.