renderer format 'json' is not valid? (why do i see this after upgrading?) (no replies)

i had some fatal errors during the recent upgrade. now though, after some tips in github, i have piwik working again.
mostly it appears fine, however, i just activated some code i recently wrote that uses piwik to identify the geo_ip details of site visitors and then displays different content to them based on their continent. whereas before the upgrade this worked fine, now i am seeing fatal PHP errors for this call. i already found one thread in the forum here related to the first error i was getting and have applied the recommended workaround to my API call to piwik:

Quote

// Workaround for 2.14.0 bug
$environment = new \Piwik\Application\Environment('tracker');
$environment->init();

the error i am now seeing appears to come after i have successfully retrieved the data from the API call:

Quote

renderer format 'json' not valid. try any of the following instead .

there are no render formats being recommended by this error code and in any case the data has been returned successfully anyway.
i changed the format to XML just to see if that made any difference, but it didn't - i still saw the same error (except with the word json replaced with XML).

anyone know what's going on here?

thanks