BUG #5001: can not prepare for where $1 is null

The following bug has been logged online:

Bug reference:      5001
Logged by:
Email address:      94487509@qq.com
PostgreSQL version: 8.3.3
Operating system:   linux
Description:        can not prepare for where $1 is null
Details:

why can not prepare like this? thanks!
 prepare ssss as select * from test where $1 is null;
 ERROR:  could not determine data type of parameter $1