The affix ka comes after a verbal root that ends in long ā when there is no upasarga preceding it and when the object is in composition with it.,
This debars अण्; as गां + दा + क = गां + द + अ = गोदः 'Giver of cows', (the long आ is elided by (VI. 4. 64,) = So also, कम्बलदः 'who gives a blanket'; पार्ष्णित्रम् 'what protects the rear'; अङ्गुलित्रम् 'what protects the finger' (a thimble)'.
Why do we say 'when there is no upasarga'? Witness गोसन्दायः 'who ceremoniously gives a cow', वडवसन्दाय 'who ceremoniously gives a mare'. Here the compound verb सन्दा takes the affix अण्.,