A Possible Misunderstanding of Modality in Kant's Groundwork

A phrase in Kant’s Grundlegung zur Metaphysik der Sitten (1785, Groundwork of the Metaphysic of Morals) that I think might have been more aptly expressed is the following:

“Alle Imperativen werden durch ein Sollen ausgedrückt” — "All imperatives are expressed by an ‘Ought'" (Chapter 2, 14^th paragraph),

which I think might have been better expressed as “Alle Imperativen werden durch ein Müssen ausgedrückt” — "All imperatives are expressed by a ‘Must.’" That is to say, if an action is imperative, it not merely “ought to” or “should” be performed, it “must” be performed, if a moral duty to perform that action is to be complied with.