Weighted Average in LookML using measures

[Palaklooker]

I have gone through 

https://community.looker.com/technical-tips-tricks-1021/how-to-make-a-weighted-average-30239

 but am facing an error. 

I have a dimension for weight, a dimension for case types and a dimension for case IDs. I want to get the weighted average for cases based on their case types. the formula i need to use is: 

weighted average = ( case_count(for case_type A)* weight% + case count(for case_type B)* weight% + case count(for case_type C)* weight% ) / (total case_count) 

dimension: weight {    # differs depending on case type associated with case
  type: number
  sql: ${TABLE}.weight ;;  
}

dimension: case_type {    # each case type has different weight
  type: string
  sql: ${TABLE}.case_type ;;  
}

dimension: case_count {
  type: count_distinct
  sql: ${TABLE}.case_id ;;
}

dimension: weighted_count {
  type: number
  sql: ${case_count} * ${weight} ;;
}

measure: weighted_average {
  type: number
  sql: sum(${weighted_count})/${request_count} ;;
 }

this gives the error “Aggregate functions cannot be nested” due to sum function in the weighted_average measure. is there a way to fix this?