action=parse & action=expandtemplates need the text parameter with the content. title doesn't automatically fetch the content from the wiki. Behavior in 1.17 is the same.
In trunk expandtemplates will give an error 'The text parameter must be set'.

API Error:

<?xml version="1.0"?>
<api servedby="srv296">

<error code="internal_api_error_MWException" info="Exception Caught: Internal error in ApiResult::setElement: Bad parameter" xml:space="preserve">

#0 /usr/local/apache/common-local/php-1.18/includes/api/ApiBase.php(1241): wfDebugDieBacktrace('Internal error ...')
#1 /usr/local/apache/common-local/php-1.18/includes/api/ApiResult.php(149): ApiBase::dieDebug('ApiResult::setE...', 'Bad parameter')
#2 /usr/local/apache/common-local/php-1.18/includes/api/ApiResult.php(180): ApiResult::setElement(Array, '*', NULL)
#3 /usr/local/apache/common-local/php-1.18/includes/api/ApiExpandTemplates.php(84): ApiResult::setContent(Array, NULL)
#4 /usr/local/apache/common-local/php-1.18/includes/api/ApiMain.php(692): ApiExpandTemplates->execute()
#5 /usr/local/apache/common-local/php-1.18/includes/api/ApiMain.php(358): ApiMain->executeAction()
#6 /usr/local/apache/common-local/php-1.18/includes/api/ApiMain.php(342): ApiMain->executeActionWithErrorHandling()
#7 /usr/local/apache/common-local/php-1.18/api.php(115): ApiMain->execute()
#8 /usr/local/apache/common-local/live-1.5/api.php(3): require('/usr/local/apac...')
#9 {main}

</error>
</api>

(In reply to comment #1)

action=parse & action=expandtemplates need the text parameter with the content.
title doesn't automatically fetch the content from the wiki. Behavior in 1.17
is the same.
In trunk expandtemplates will give an error 'The text parameter must be set'.

That's true and I usually do it that way, but this time I did it that way cause the text={{... form which I tried first didn't work. So both ways produce empty pages, but only the title one a PHP error, dunno if that's caused by the page or is another error (but I'm sure it shouldn't be that way)

Tagged r93977 for merging to fix the backtrace.

(In reply to comment #4)

Tagged r93977 for merging to fix the backtrace.

Isn't it's prequisite also needed?

Has been fixed as it seems (tested with examples out of the first comment)