وَكُلُّ مَا لَا يَنْصَرِفُ، إِذَا أُضِيفَ أَوْ دَخَلَهُ اللَّامُ، فَيَدْخُلُهُ الْكَسْرَةُ.
Translation: When a diptote enters an iḍāfah construction or takes the definite article al-, it receives kasrah (in the genitive case).
The rule concerns the genitive ending, not the use of kasrah in every syntactic position.
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Restoring Declension: Indefinite Names, Al- and Iḍāfah
وَكُلُّ مَا لَا يَنْصَرِفُ، إِذَا أُضِيفَ أَوْ دَخَلَهُ اللَّامُ، فَيَدْخُلُهُ الْكَسْرَةُ.
Translation: When a diptote enters an iḍāfah construction or takes the definite article al-, it receives kasrah (in the genitive case).
The rule concerns the genitive ending, not the use of kasrah in every syntactic position.