Problem with SUM on Matrix blocks

I have some Matrix blocks where I need to sum one of the fields within, but I only want to sum it when another field within that block is or is not empty.

{% set reimbursementAmount = craft.matrixBlocks()
  .relatedTo(cat)
  .fieldId(9)
  .sum('field_item_reimbursementAmount')
  .specialCase(':empty:')
  %} 

With this, I get an error:

Impossible to invoke a method ("specialCase") on a string variable ("101.00").

How do I sum the desired field, but only where another field (specialCase) is empty (or is not empty, as the case may be).

{% set reimbursementAmount = craft.matrixBlocks()
  .relatedTo(cat)
  .fieldId(9)
  .specialCase(':empty:')
  .sum('field_item_reimbursementAmount')
%}