Error: Cannot assign to non-existent property

[ustulation]

main.qml:

import QtQuick 2.0
 
Rectangle {
    height: 300
    width: 300
    color: "black"
 
    Zero {
        m_ref.m_url: "something"
    }
}

import QtQuick 2.0

Rectangle {
    height: 300
    width: 300
    color: "black"

    Zero {
        m_ref.m_url: "something"
    }
}

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Zero.qml

import QtQuick 2.0
 
Item {
    property alias m_ref: objZero
 
    Rectangle {
        id: objZero
 
        property url m_url
    }
}

import QtQuick 2.0

Item {
    property alias m_ref: objZero

    Rectangle {
        id: objZero

        property url m_url
    }
}

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This gives an error:

"m_url" Cannot assign to non-existent property
             m_ref.m_url: "something" 
                   ^

"m_url" Cannot assign to non-existent property
             m_ref.m_url: "something" 
                   ^

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However i can use m_url inside Zero.qml anywhere (by qualifying with the correct id ofcourse) or alias it with the topmost Item of Zero.qml and use the alias in main.qml.
Why using it the way shown above gives an error while the other methods (in the previous sentence) don't?

{using Qt 5.1.0 beta and Rc both, Windows 7, QtQuick 2.0}