Weird problem with aggregate functions

Hi everybody,
I got a problem with aggregate functions from VB 6. SUM, AVG, STDDEV,
and VARIANCE doesn't work on int4 types (but they're working on float8),
whereas MAX and MIN work also on int4... that's weird!
Even more strange, the above functions work both from psql and from
pgAdmin II... but not in the following code:

Private Sub Form_Load()
  Dim conn As ADODB.Connection
  Dim rec As ADODB.Recordset
  Dim cmd As Command

  Set conn = New Connection
  conn.CursorLocation = adUseClient
  conn.Open "DSN=DatabaseSIA"

  Set rec = New Recordset

  rec.Open "Select SUM(popolazione) from tab2531", conn, adOpenStatic,
adLockReadOnly
  Me.Print rec.Fields(0).Name & ": " & rec.Fields(0).Value
  rec.Close
End Sub

where tab2531 is defined as follows:

CREATE TABLE "tab2531" (
  "abbreviazione" text,
  "regione" int4,
  "popolazione" int4,
  "densita" int4,
  "natalita" float8,
  "mortalita" float8,
  "mortalinfant" float8,
  "nuzialita" float8
);

The program halts with this error (translated from italian): "data
provider
or another service returned E_FAIL"
I tested this with 7.01.0008 and 7.01.0007 both on WinNT and Win2000...
same results. I also played with driver options. commlog doesn't report
any error. Ah, I use ADO 2.6 (tested also with 2.5, same results)
Any idea?
Best regards
Andrea Aime