QPlainTextEdit: setInputMethodHints(Qt::ImhDigitsOnly); has no effect

I am using Qt 5.1.1 with Visual Studio 2012 and Qt Creator 2.8.1.
I want to allow only digits within my QPlainTextEdit so I am using the following method:

Code:

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ui->plainTextEdit->setInputMethodHints(Qt::ImhDigitsOnly);

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However, this has no effect. I can still enter characters :confused: