Parse message problem, maybe a bug?

Hi all,

Martijn, a user of Npgsql, sent me a query which is giving problems with 
postgresql using extended query mode.

The problem I'm having is in the Parse message. The parse message I'm 
sending is as follow:


select * from table where $1 in (select some_field from table)

Postgresql returns this error:

could not determine data type of parameter $1
Severity: ERROR
Code: 42P18


But I already have many working samples in the form for example:

insert into table(field1, field2) values ($1, $2)


The only way I could have it working was by adding the explicit type of 
parameter so the parse text is, for example for parameter of type text:


select * from table where $1::text in (select some_field from table)


Is this a bug in Postgresql or is this by design?


Thanks in advance.


Regards,

Francisco Figueiredo Jr.