PL/Python problem

I am using the latest cygwin build of postgresql and python
I used this code to create a trigger:

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CREATE FUNCTION negaterow() RETURNS trigger AS
'if TD["new"]["a3a"]!=None and TD["new"]["assmt_in_id"]!=None:
TD["new"]["nega3a"] = 0-TD["new"]["a3a"]
TD["new"]["negassmt_in_id"] = 0-TD["new"]["assmt_in_id"]
return "MODIFY"
return None'
LANGUAGE 'plpython';

CREATE TRIGGER negate
BEFORE INSERT ON mdsqiitems1
FOR EACH ROW
EXECUTE PROCEDURE negaterow();

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Whenever I insert a row:
If no string in the data is longer that length 1, it works fine.
If a string is longer, it says
"ERROR: value too long for type character varying(1)".

But:
a) The fields in question are _not_ varchar(1).
b) It works fine without the trigger.

What's going on?

-Bryan Klimt