Found entity of type Doctrine\Common\Collections\ArrayCollection on association AppBundle\Entity\Appareil#combustibles, but expecting AppBundle\Entity\Combustible

Appareil.php

  <?php

namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
 * Appareil
 *
 * @ORM\Table()
 * @ORM\Entity
 */
class Appareil
{  /**
     * @ORM\ManyToOne(targetEntity="Combustible" , inversedBy="Appareil")
     * @ORM\JoinColumn(name="combustibles",referencedColumnName="id")
     */
    private $combustibles;
    public function addCombustible(Combustible $combustible)
    {
        if (!$this->combustible->contains($combustible)) {
            $this->combustible->add($combustible);
        }
    }


    public function __toString() {
        return $this->name;
    }

    /**
     * Set combustibles
     *
     * @param \AppBundle\Entity\Combustible $combustibles
     * @return Appareil
     */
    public function setCombustibles( $combustibles = null)
    {

        $this->combustibles[] = $combustibles;

        return $this;
    }

    /**
     * Get combustibles
     *
     * @return \AppBundle\Entity\Combustible 
     */
    public function getCombustibles()
    {
        return $this->combustibles;
    }
    }

AppareilAdmin.php

<?php
class AppareilAdmin extends Admin
{
    protected $baseRouteName = 'AppareilAdmin';
    protected $baseRoutePattern = 'AppareilAdmin';

    protected function configureFormFields(FormMapper $formMapper)
    {
        $formMapper->add('name', 'text')
            ->add('description', 'textarea')
            ->add('combustibles', 'entity', array('class' => 'AppBundle\Entity\Combustible', 'multiple' => true, 'expanded' => true, 'property' => 'name'))
            ->add('media', 'sonata_media_type', array('provider' => 'sonata.media.provider.image'));
    }

    protected function configureDatagridFilters(DatagridMapper $datagridMapper)
    {
        $datagridMapper->add('name')->add('description')->add('combustibles');
    }

    protected function configureListFields(ListMapper $listMapper)
    {
        $listMapper->addIdentifier('name')->add('description')->add('combustibles');
    }
}

Combustible.php

<?php 
/**
 * Combustible
 *
 * @ORM\Table()
 * @ORM\Entity
 */
class Combustible
{
    /**
     * @ORM\OneToMany(targetEntity="Appareil" , mappedBy="Combustible")
     */

    private $appareils;
    /**
     * Add appareils
     *
     * @param \AppBundle\Entity\Appareil $appareils
     * @return Combustible
     */
    public function addAppareil(\AppBundle\Entity\Appareil $appareils)
    {
        if (!$this->appareils->contains($appareils)) {
            $this->appareils->add($appareils);
        }
        return $this;
    }

    /**
     * Remove appareils
     *
     * @param \AppBundle\Entity\Appareil $appareils
     */
    public function removeAppareil(\AppBundle\Entity\Appareil $appareils)
    {
        $this->appareils->removeElement($appareils);
    }

    /**
     * Get appareils
     *
     * @return \Doctrine\Common\Collections\Collection
     */
    public function getAppareils()
    {
        return $this->appareils;
    }
}

Adding Many to one relation between tuo entities problem