category when using "include_once PATH_MOD.'channel/mod.channel'.EXT"

I have a plugin that uses

include_once PATH_MOD.'weblog/mod.weblog'.EXT;

in EE1 or

include_once PATH_MOD.'channel/mod.channel'.EXT;

in EE2 to display entries. But when using it in EE2, the category parameter in the plugin breaks the page. The response is “Error: The query you submitted is not valid.” This happens quite early on as neither Display Output Profiler nor Display Template Debugging will work.

Can anybody tell me if there’s a way to do this that works?