Meter:Custom is confused about units

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I've used Meter:Custom for energy in the past, but never used it for a typical Output:Variable. I'm trying to aggregate all of the outdoor airflows of a bunch of PTACs.

When I do so, this warning below results - and none of the airflows are added to the meter. Can one of you add some light here? Thanks in advance.

The object looks like:

Meter:Custom,
  OAflow,                  !- Name
  Generic,                 !- Fuel Type
  Floor1:NWTHCorner PTAC Outdoor Air Node Name,  !- Key Name 1
  System Node Standard Density Volume Flow Rate,  !- Output Variable or Meter Name 1

.... etc for several nodes....

Here's the warning (repeated for all 20+ nodes)

** Warning ** DetermineMeterIPUnits: Meter units not recognized for IP Units conversion=[m3/s].
**   ~~~   ** ..on Meter:Custom="OAFLOW".
**   ~~~   ** ..requests for IP units from this meter will be ignored.
** Warning ** Meter:Custom="OAFLOW", variable not summed variable Output Variable or Meter Name="SYSTEM NODE STANDARD DENSITY VOLUME FLOW RATE".
**   ~~~   ** ...will not be shown with the Meter results; units for meter=m3/s, units for this variable=m3/s.