Arrays and count()

I'm having difficulties formulating what I thought was a simple
query in 6.3.2. I have a table defined like this:

create table sentence
(
typenum int4,
wordpos int4,
words text[],
tags text[]
);

What I'd like to do is, e.g. select all instances of tags[1] and
display the frequency values for each unique (distinct) entry.
The query I tried was:

select sentence.tags[1], count (*) from sentence group by
sentence.tags[1];

Which fails to parse. Trying to work backwards I try constructing a
related command:

select count (sentence.tags[1]) from sentence;

And am treated to the error message:

PQexec() -- Request was sent to backend, but backend closed the channel
before responding.
This probably means the backend terminated abnormally before or
while processing the request.

Is this a bug, or am I completely failing to understand one or more
of the concepts involved here?

Mike Pacey.