json_decode mistery

Hello, I am usin json regulary, but in one script I have mistery:

echo($decrypted_data)."\n\n";
var_dump(json_decode($decrypted_data, true));
echo "\n";
var_dump(json_decode('{"result_ok":true,"result_message":null,"client_name":"Radek Krej\u010da"}', true));

I got:

{"result_ok":true,"result_message":null,"client_name":"Radek Krej\u010da"}

NULL

array(3) {
  ["result_ok"]=>
  bool(true)
  ["result_message"]=>
  NULL
  ["client_name"]=>      a"
} string(13) "Radek KrejÄ


You can see, that in $decrypted_data, is stored valid (by my opinion) json data. If I use this variable in json_decode, I got "null". And if I manualy use data displayed on screen, I got valid array.

Where I do mistake? If I remove client_name (so no utf8 is in json data), situation is the same.

Radek