"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