While Applying Math Expression for elastic dashboard filter query to reduce and get sum, returning zero

We are facing problem with
dashboard filter when applied for individual widget when more computation are required

Without - Math Expression Reduce : Returning value for a specific dashboard filter
message_extract.data.*******.keyword:$cam

With - Math Expression Reduce : No value for a specific dashboard filter
message_extract.data.*******.keyword:$cam

Note : But it works fine for other filter
Eg
message_extract.data.*********.keyword:“hello”

Please let me know how to solve this
Thanks in Advance!