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;

SCAN($B$4:$F$4,{"cat","dog"}, LAMBDA(x,y, SUM( --ISNUMBER(SEARCH(y,x)))))

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;

=

BYROW( B4:F10, LAMBDA(A,REDUCE(0,A, LAMBDA(x,y, x + SUM(--ISNUMBER(SEARCH({"cat","dog"}, y)))))>0 ))

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

Richard.