Problem With Recursion (categories and subcategories)

Question

I'm testing a code from the answer by @digitalsteez on this question:

Get Categories and Its Sub Categories

I've tested it onto two Magento instance, one has no problems but on the other I'm facing a problem with recursion:

"children" => getChildCategories($child)

and

renderCategoriesTree($level, $child);

output me an

"Invalid argument supplied for foreach()"

How can I solve this?

Thank You in advice.