[O] problem with empty column

[Uwe Brauer]

Hi

Consider please the following table



| Name   | E1 | E2 | E3 | E4 | Res |
|--------+----+----+----+----+-----|
| Entry1 |    |    |    |    | NP  |
| Entry2 | 10 | 20 | 30 | 40 | 10  |
#+TBLFM: $6=if("$2" == "nan" , string("NP"),($2+$3+$4+$5)/10);E

The last column contains a string NP if the corresponding cell in the
second column is empty.

However when   I consider

| Name   | E1 | E2 | E3 | E4 | Res |
|--------+----+----+----+----+-----|
| Entry1 |    |    |    |    |  NP |
| Entry2 | 10 | 20 | 30 | 40 |  10 |
| Entry3 | 10 |    | 20 | 30 | nan |
#+TBLFM: $6=if("$2" == "nan" , string("NP"),($2+$3+$4+$5)/10);E


The last row behaves strangely the cell in the second column is *not*
empty but the cell in the third one is, nevertheless the cell in the
last column contains the string nan, which I don't understand.

Any help would be appreciated

Thanks

Uwe Brauer