Kallā as a rebuke is indeclinable and non-governing. A reported nominal reading also treats it as indeclinable. An indeclinable noun need not display changing case vowels.
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Word Class and Case Are Distinct
The Particle Kallā: Rejection, Restraint and Emphasis
Kallā as a rebuke is indeclinable and non-governing. A reported nominal reading also treats it as indeclinable. An indeclinable noun need not display changing case vowels.