Expression in query

[Radhika]

I am trying to create a query form another query which takes the sum of two
fields and the count of a third. The query I am trying to create should adds
the 'sum' fields and divided the result by the 'count' field. Below is my
expression I am using:

Average: (([SumOfProcedures non-time based]+[SumOfTime Based
Units])/[CountOf# Hours Worked])

However, I get '#Error' as my result. This does not happen if I only add the
two sum fields:
([SumOfProcedures non-time based]+[SumOfTime Based Units])

What am I doing incorrectly?

Thank you,
Radhika