Operand data type NULL is invalid for sum operator

Hi everyone,

I'm running into a power bi dataflow refresh issue: Operand data type NULL is invalid for sum operator.

i think this issue may link to a groupby step in the flow for several decimal columns with null values included. 

it looks fine when i finish the groupby and save the flow. but such error blocks me from refrehsing the dataflow.

Could you pls help advise the real reason? thank you.