Currently the assertion is:
Q_ASSERT(uint < uint(size()))

With this the '\0' cannot retrieved with at() or []
will be good to change this to:
Q_ASSERT(uint <= uint(size()))

Ex:
QString echoChar(QChar c)

{ if (c == QChar(0)) return "<NUL>"; else ... return QString(c); }

QString test("01234"); // size() == 5

qDebug() << "test[0]: " << echoChar(test[0]); // output: test[0]: "0"
qDebug() << "test[1]: " << echoChar(test[1]); // output: test[1]: "1"
qDebug() << "test[2]: " << echoChar(test[2]); // output: test[2]: "4"
qDebug() << "test[5]: " << echoChar(test[5]); // expected output: test[5]: "NUL",
but here the assertion will be fired!

I fink that code are 'copied' from QByteArray that doen't have null terminate char.