problem with using http_post where http_get worked

Using both SWIPL 6.4.1 and 6.5.2.

I had code which was working with http_get:

http_get(URL, Response, [content_type('text/xml'), space(sgml), 
proxy('192.168.121.254', 3128)]).

But then I realized I needed to use post and add a parameter.  According to the 
documentation http_post is equivalent to http_get, but with the extra parameter 
so I tried:

http_post(URL, form_data([ searchLetter = 'C' ]), Response, 
[content_type('text/xml'), space(sgml), proxy('192.168.121.254', 3128)]).

This returns an error I can't make sense of:
ERROR: Domain error: `sgml_option' expected, found `proxy(192.168.121.254,3128)'

Any clues?
Steve