"no snapshot has been set" error

Hi all,

I encountered a problem when I'm implementing my user-defined type.
My user-defined type is defined as

CREATE TYPE X
(
INTERNALLENGTH = 10
INPUT = X_in,
OUTPUT = X_out
);

In my X_in() function, I want to access other table to do some
bookkeeping stuff. this table keeps track of the number of calls to
X_in(). However, while SPI_connect() returns SPI_OK_CONNECT, whenever I
execute SPI_exec( "SELECT count FROM XTABLE", 0), postgres stops
executing and issues "Error: no snapshot has been set".

Does anyone here knows what happens? Am I not allowed to use SPI
functions in this situation?

Thank you for your advice and suggestions in advance.

Ruey-Lung Hsiao