使用PHP格式错误的JSON

I am a .NET guy with fairly long experience but recently pulled into a PHP project so need some help. I have this PHP code which prepares a final song object and then JSONifies it with json_encode() but I strangly I am seeing some extra value null added at the end of JSON string and I dont seem to understand why is that.

Here is my PHP code

foreach($result as $item) {
    ++$index;
    $hymn->id=$item['id'];
    $hymn->refrain=$item['refrain'];

    if ($index == 1) { $hymn->stanza1=$item['stanzaText']; }
    if ($index == 2) { $hymn->stanza2=$item['stanzaText']; }
    if ($index == 3) { $hymn->stanza3=$item['stanzaText']; }
    if ($index == 4) { $hymn->stanza4=$item['stanzaText']; }
    if ($index == 5) { $hymn->stanza5=$item['stanzaText']; }
    if ($index == 6) { $hymn->stanza6=$item['stanzaText']; }
    if ($index == 7) { $hymn->stanza7=$item['stanzaText']; }
    if ($index == 8) { $hymn->stanza8=$item['stanzaText']; }
    if ($index == 9) { $hymn->stanza9=$item['stanzaText']; }
    if ($index == 10) { $hymn->stanza10=$item['stanzaText']; }
}

json_encode($hymn);

Above code produces a JSON with extra null after the ending curly braces as shown below.

{
    "id":"1",
    "refrain" : "Jesus, Jesus, how I trust Him!
How I\u2019ve proved   Him o\u2019er and o\u2019er;
Jesus, Jesus, precious Jesus!
Oh, for grace to trust Him more!",
    "stanza1" : "Tis so sweet to trust in Jesus,
Just to take Him at His Word;
Just to rest upon His promise,
And to know, \u201cThus saith the Lord!\u201d",
    "stanza2" : "Oh, how sweet to trust in Jesus,
Just to trust His cleansing blood;
And in simple faith to plunge me
\u2019Neath the healing, cleansing flood!",
    "stanza3" : "Yes, \u2019tis sweet to trust in Jesus,
Just from sin and self to cease;
Just from Jesus simply taking
Life and rest, and joy and peace.",
    "stanza4" : "I\u2019m so glad I learned to trust Thee,
Precious Jesus, Savior, Friend;
And I know that Thou art with me,
Wilt be with me to the end"
 }null

Kindly help me to understand what wrong am I doing here?

All friends, I apologize as I should have made this an answer before but problem is not with JSON. It is with fetchAll(). Thanks for analyzing and helping me.

wrong. json_encode does NOT create that null at the end, something else, after this code, does. show us the rest of the code, and we might be able to spot what's making it.