Do you have an AirLoopHVAC that serves no zones? Or zones that have zero area?

From AirLoopHVAC#L349:

OpenStudio.logFree(OpenStudio::Debug, 'openstudio.standards.AirLoopHVAC', "For #{name}: flow per area = #{cfm_per_ft2.round} cfm/ft^2.")

And if you go up a little:

# Calculate and report the total area for debugging/testing
floor_area_served_m2 = floor_area_served
floor_area_served_ft2 = OpenStudio.convert(floor_area_served_m2, 'm^2', 'ft^2').get
cfm_per_ft2 = dsn_air_flow_cfm / floor_area_served_ft2

AirLoopHVAC#L3490

# Calculate the total floor area of all zones attached
 # to the air loop, in m^2.
 #
 # return [Double] the total floor area of all zones attached
 # to the air loop, in m^2.
 def floor_area_served
   total_area = 0.0

   thermalZones.each do |zone|
     total_area += zone.floorArea
   end

   return total_area
end