PHP : Undefined property: stdClass::

PHP : Undefined property: stdClass::

I've got this error message and I can't seem to find the reason for it. I
use CodeIgniter as my php framerwork.
A PHP Error was encountered
Severity: Notice
Message: Undefined property: stdClass::$HP
Filename: helpers/battle_helper.php
Line Number: 13
In my battle_helper.php I have this 2 functions :
function calculateStats($nature, $EV, $IV, $level){
$base = calculateBaseStats();
$stats = new stdClass();
$stats->HP = (($IV->HP + 2 * $base->HP + ($EV->HP / 4)) * $level /
100) + 10 + $level;
$stats->ATTACK = ((($IV->ATTACK + 2 * $base->ATTACK + ($EV->ATTACK /
4)) * $level / 100) + 5) * $nature->attack;
$stats->DEFENCE = ((($IV->DEFENCE + 2 * $base->DEFENCE + ($EV->DEFENCE
/ 4)) * $level / 100) + 5) * $nature->defence;
$stats->SPCATTACK = ((($IV->SPCATTACK + 2 * $base->SPCATTACK +
($EV->SPCATTACK / 4)) * $level / 100) + 5) * $nature->spcattack;
$stats->SPCDEFENCE = ((($IV->SPCDEFENCE + 2 * $base->SPCDEFENCE +
($EV->SPCDEFENCE / 4)) * $level / 100) + 5) * $nature->spcdefence;
$stats->SPEED = ((($IV->SPEED + 2 * $base->SPEED + ($EV->SPEED / 4)) *
$level / 100) + 5) * $nature->speed;
return $stats;
}
function calculateBaseStats()
{
$base = new stdClass();
$base->HP = 0;
$base->ATTACK = 0;
$base->DEFENCE = 0;
$base->SPCATTACK = 0;
$base->SPCDEFENCE = 0;
$base->SPEED = 0;
return $base;
}
When I echo the $base->HP inside the calculateStats function. I get the
value 0 on my webpage without the error. But the error still remains on
line 13.