postgres 9.5 create function plpthon3u resets connections to server

postgresql 9.5 on windows 10, x64.

I have created the extension plpython3u with python 3.3.5 on the server's path and it appeared to create the extension successfully:

SELECT * FROM pg_available_extensions
WHERE name like '%python%' order by name;

      name        | default_version | installed_version |                  comment

-------------------+-----------------+-------------------+------------------------------------------
-hstore_plpython2u | 1.0             |                   | transform between hstore and plpython2uhstore_plpython3u | 1.0             |                   | transform between hstore and plpython3uhstore_plpythonu  | 1.0             |                   | transform between hstore and plpythonultree_plpython2u  | 1.0             |                   | transform between ltree and plpython2ultree_plpython3u  | 1.0             |                   | transform between ltree and plpython3ultree_plpythonu   | 1.0             |                   | transform between ltree and plpythonuplpython2u        | 1.0             |                   | PL/Python2U untrusted procedural languageplpython3u        | 1.0             | 1.0               | PL/Python3U untrusted procedural languageplpythonu         | 1.0             |                   | PL/PythonU untrusted procedural language
(9 rows)

However when I attempt to create the following function (from the pg docs)

CREATE FUNCTION pymax (a integer, b integer) RETURNS integer
AS $$ if a > b:   return a return b
$$ LANGUAGE plpython3u;

the psql (or pgadmin3) terminal's connection is reset.

The python 3.3 on the path is anaconda's distb and runs fine on its own. I couldn't find the required version of python in the postgresql docs and used dependency walker as described here Postgres database crash when installing plpython to find the required dll that plpython3.dll in the server's lib/ requires.

Can anyone help me with what I have missed?

This question is also posted on stackoverflow here. 

Thanks in advance for your help.

Many thanks