QFileDialog::getSaveFileName Save Button

[penny]

I'm using QFileDialog::getSaveFileName to save a file in my application

QString fileName=QFileDialog::getSaveFileName(this,"Save As");

QString fileName=QFileDialog::getSaveFileName(this,"Save As");

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However, in the dialog box , the SAVE button is always enabled, even when no text is written in save as file name text box.
I want the save button to get enabled only after a file name i written in the text box.
How do i do that?