CALC Error within LAMBDA

Hello, can someone explain or point me in the direction of information ;

I have a Lambda formula to sum the count of cat or dog for each row;

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that will spill the sum for cat and the sum of count for dog, but if i remove the SUM part I just get a CALC error,
i could understand it not working at all, but find this difficult, I have had a few similar with lambda and vistack where there is an error until wrapped in sum,

as it turned out it was not the right approach anyway and I needed; REDUCE;

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as said i'm just interested in why the error occurs until wrapped in SUM, I have attached the work book with examples.

Richard.

Thanks, that makes perfect sense. I'll have a play around with your suggestion.

Richard.