Problem with array and cyrillic

[eluser]Unknown[/eluser]
Hello I have this array

Code:

$set = array(
'text' => 'Текст на кирилица'      
);

and I return the result like this

Code:

echo json_encode($set);

But the result is

{"text" : null}

But when I change text to 'Latin text' everything is alright...It returns me

{"text" : "Latin text"}

So my question why the array doesn't allow Cyrillic values and how can I fix this problem?