Power query zero versus null in calculaton

Hi I have a question regarding the use of null,

if I have N columns containing numbers and zeros and multiply them I end up with 0 , all good,
but if I want he 0's ignored I can change these to null values and then,

List.Product({[Number], [One], [Two], [Three]}), type number)

but have tried an accumulation;

 Table.AddColumn( Source, "new",  each 
 List.Accumulate( ColNames, 0, (S,C)=> 
 S + Record.Field( MMM {0} , C ) * Record.Field(_, C) * [Number] ) )

in the above each column contains 1 or 0 , and another table values 2,3,4, and this works, but if I change the 0's to nulls
any row containing a null returns a total of null?
Why does this happen,

I have attached the sheet with bot versions, I have a further questing regarding null but will leave that for later.

Richard