Array in PHP,Json

I have a problem with array in php at json_encode.

I have 2 arrays that work in PHP but only $ testArray works when converting to json_encode.

When output in array in php everything works as it should. But when converting to json, only one works.

Output is the same for both.

if ($result->num_rows > 0) {
while($row = $result->fetch_assoc()) {
    if($row["Autor"] !== "" && $row["Nazev"] !== "")
    {
    $knih_data = [
    "Autor" => "".$row["Autor"]."",
    "Nazev" => "".$row["Nazev"]."",
    "Poznamka" => "".$row["Poznamka"]."",    
    ];
    $knihovna[] = $knih_data;    
    }
}
}





$testArray = [
    [
      "Autor"   => "Autor",
      "Nazev" => " Testuju"
    ],
    [
      "Autor"   => "Autor 2",
      "Nazev" => "Testuju..2",
     "Poznamka" => "Text.. 2"
    ],
    [
      "Autor"   => "Autor 3",
      "Nazev" => "Testuju 3",
      "Poznamka" => "Text.. 3"
    ]
  ];

print_r($testArray); //Work
print_r($knihovna); //Work



  $json_convert = json_encode($testArray);//Work
  echo $json_convert;

  $json_convert = array_values($knihovna);//Notwork
  echo $json_convert;

Output

  Array//testArray
(

[0] => Array
    (
        [Autor] => Někdo
        [Nazev] =>  Zlep�ov�n� podnikov�ch proces
    )

[1] => Array
    (
        [Autor] => Někdo 2
        [Nazev] => Testuju..2
        [Poznamka] => Text.. 2
    )

[2] => Array
    (
        [Autor] => Někdo 3
        [Nazev] => Testuju 3
        [Poznamka] => Text.. 3
    )

)

Array //Knihovna
(

[0] => Array
    (
        [Autor] => Jan Urban
        [Nazev] => 10 nejdra���ch mana�ersk�ch ch
        [Poznamka] => 
    )

[1] => Array
    (
        [Autor] => Forbes
        [Nazev] => ?�slo 3/2013
        [Poznamka] => 
    )

[2] => Array
    (
        [Autor] => Jaroslav Charv�t
        [Nazev] => Firemn� strategie pro praxi
        [Poznamka] => 
    )

[3] => Array
    (
        [Autor] => Nierenberg, Calero, Grayson
        [Nazev] => How to read a person like a bo
        [Poznamka] => 
    )

[4] => Array
    (
        [Autor] => Jim Collins, Jerry I. Porras
        [Nazev] => Jak vybodovat trvale �sp?�nou 
        [Poznamka] => 
    )

[5] => Array
    (
        [Autor] => Miller, Wrobleski, Villafuerte
        [Nazev] => Kultura kaizen
        [Poznamka] => 
    )

[6] => Array
    (
        [Autor] => Simon Synek
        [Nazev] => L�d?i jed� posledn�
        [Poznamka] => 
    )

[7] => Array
    (
        [Autor] => Dan Roam
        [Nazev] => N�pady na ubrousku: ?e�te prob
        [Poznamka] => 
    )

[8] => Array
    (
        [Autor] => Chris Voss, Tahl Raz
        [Nazev] => Nikdy ned?lej kompromis
        [Poznamka] => 
    )

[9] => Array
    (
        [Autor] => Keith Ferrazzi
        [Nazev] => Nikdy nejez s�m
        [Poznamka] => 
    )

[10] => Array
    (
        [Autor] => Simon Synek
        [Nazev] => Objevte sv� pro?
        [Poznamka] => 
    )

[11] => Array
    (
        [Autor] => Michal Martoch 
        [Nazev] => ?�zen� vzd�len�ch pracovn�k?
        [Poznamka] => 
    )

[12] => Array
    (
        [Autor] => Roman Baj?an
        [Nazev] => Techniky public relations aneb
        [Poznamka] => 
    )

[13] => Array
    (
        [Autor] => Alena Svozilov�
        [Nazev] => Zlep�ov�n� podnikov�ch proces?
        [Poznamka] => 
    )

)

There is all ouput in array with print_R