Extended query: parse command freezes backend

I'm sending this parse command after which the backend completely
stops responding (just the session, not the whole server):

   Byte1('P'): "P"
   Int32:       41
   String:     "mystatement" + null byte
   String:     "select * from mytable;" + null byte
   Int16:       0

I have to close and re-open the frontends' TCP/IP connection to be
able to work again.  PostgreSQL 7.4.2.

Marc