GameObject has undefined tag!

In m_MaterialHit = hit.collider.transform.tag; error!!! Code:

 using UnityEngine;
 using System.Collections;
 
 public class bl_FootSteps : Photon.MonoBehaviour
 {
 
     public bool CanUpdate = true;
     public AudioSource InstanceReference;
     public AudioClip[] m_dirtSounds;
     public AudioClip[] m_concreteSounds;
     public AudioClip[] m_WoodSounds;
     public float m_minSpeed = 2f;
     public float m_maxSpeed = 8f;
     public float audioStepLengthCrouch = 0.65f;
     public float audioStepLengthWalk = 0.45f;
     public float audioStepLengthRun = 0.25f;
     public float audioVolumeCrouch = 0.3f;
     public float audioVolumeWalk = 0.4f;
     public float audioVolumeRun = 1.0f;
     public PhotonView m_view;
     //private
     private bool isStep = true;
     private string m_MaterialHit;
 
     // Update is called once per frame
     void Update()
     {
         if (!CanUpdate)//if remote is not updated, only receives the RPC call (optimization helps)
             return;
         if (InstanceReference == null)
             return;
 
         RaycastHit hit;
         if (Physics.Raycast(transform.position, -Vector3.up, out hit, 10))
         {
             m_MaterialHit = hit.collider.transform.tag;
         }
         float m_magnitude = m_charactercontroller.velocity.magnitude;
         if (m_charactercontroller.isGrounded && isStep)
         {
             switch (m_MaterialHit)
             {
                 case "Concrete":
                     if (m_magnitude < m_minSpeed && m_magnitude > 0.75f)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Crouch", "Concrete");
                     }
                     else if (m_magnitude > m_minSpeed && m_magnitude < m_maxSpeed)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Walk", "Concrete");
                     }
                     else if (m_magnitude > m_maxSpeed)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Run", "Concrete");
                     }
                     break;
                 case "Dirt":
                     if (m_magnitude < m_minSpeed && m_magnitude > 0.75f)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Crouch", "Dirt");
                     }
                     else if (m_magnitude > m_minSpeed && m_magnitude < m_maxSpeed)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Walk", "Dirt");
                     }
                     else if (m_magnitude > m_maxSpeed)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Run", "Dirt");
                     }
                     break;
 
                 case "Wood":
                     if (m_magnitude < m_minSpeed && m_magnitude > 0.75f)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Crouch", "Wood");
                     }
                     else if (m_magnitude > m_minSpeed && m_magnitude < m_maxSpeed)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Walk", "Wood");
                     }
                     else if (m_magnitude > m_maxSpeed)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Run", "Wood");
                     }
                     break;
                 default:
                     if (m_magnitude < m_minSpeed && m_magnitude > 0.75f)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Crouch", "Concrete");
                     }
                     else if (m_magnitude > m_minSpeed && m_magnitude < m_maxSpeed)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Walk", "Concrete");
                     }
                     else if (m_magnitude > m_maxSpeed)
                     {
                         m_view.RPC("SyncSteps", PhotonTargets.All, "Run", "Concrete");
                     }
                     break;
             }            
         }
     }
 
     IEnumerator Crouch(string m_material)
     {
         if (InstanceReference.GetComponent<AudioSource>() == null)
             yield return null;
 
         isStep = false;
         switch (m_material)
         {
             case "Dirt":
                 InstanceReference.clip = m_dirtSounds[Random.Range(0, m_dirtSounds.Length)];
                 InstanceReference.volume = audioVolumeCrouch;
                 InstanceReference.GetComponent<AudioSource>().Play();
                 break;
             case "Concrete":
                 InstanceReference.clip = m_concreteSounds[Random.Range(0, m_concreteSounds.Length)];
                 InstanceReference.volume = audioVolumeCrouch;
                 InstanceReference.GetComponent<AudioSource>().Play();
                 break;
             case "Wood":
                 InstanceReference.clip = m_WoodSounds[Random.Range(0, m_WoodSounds.Length)];
                 InstanceReference.volume = audioVolumeCrouch;
                 InstanceReference.GetComponent<AudioSource>().Play();
                 break;
 
         }
         yield return new WaitForSeconds(audioStepLengthCrouch);
         isStep = true;
     }
 
 
 
     IEnumerator Walk(string m_material)
     {
         if (InstanceReference.GetComponent<AudioSource>() == null)
             yield return null;
 
         isStep = false;
         switch (m_material)
         {
             case "Dirt":
                 InstanceReference.clip = m_dirtSounds[Random.Range(0, m_dirtSounds.Length)];
                 InstanceReference.volume = audioVolumeWalk;
                 InstanceReference.GetComponent<AudioSource>().Play();
                 break;
             case "Concrete":
                 InstanceReference.clip = m_concreteSounds[Random.Range(0, m_concreteSounds.Length)];
                 InstanceReference.volume = audioVolumeWalk;
                 InstanceReference.GetComponent<AudioSource>().Play();
                 break;
             case "Wood":
                 InstanceReference.clip = m_WoodSounds[Random.Range(0, m_WoodSounds.Length)];
                 InstanceReference.volume = audioVolumeWalk;
                 InstanceReference.GetComponent<AudioSource>().Play();
                 break;
 
         }
         yield return new WaitForSeconds(audioStepLengthWalk);
         isStep = true;
     }
 
     IEnumerator Run(string m_material)
     {
         if (InstanceReference.GetComponent<AudioSource>() == null)
            yield return null;
 
         isStep = false;
         switch (m_material)
         {
             case "Dirt":
         InstanceReference.clip = m_dirtSounds[Random.Range(0, m_dirtSounds.Length)];
         InstanceReference.volume = audioVolumeRun;
         InstanceReference.GetComponent<AudioSource>().Play();
                 break;
             case "Concrete":
         InstanceReference.clip = m_concreteSounds[Random.Range(0, m_concreteSounds.Length)];
         InstanceReference.volume = audioVolumeRun;
         InstanceReference.GetComponent<AudioSource>().Play();
                 break;
             case "Wood":
         InstanceReference.clip = m_WoodSounds[Random.Range(0, m_WoodSounds.Length)];
         InstanceReference.volume = audioVolumeRun;
         InstanceReference.GetComponent<AudioSource>().Play();
                 break;
 
         }
         
         yield return new WaitForSeconds(audioStepLengthRun);
         isStep = true;
     }
 
     [PunRPC]
     void SyncSteps(string t_corrutine,string m_material)
     {
         StartCoroutine(t_corrutine,m_material);
     }
 
     public void OffUpdate()
     {
         CanUpdate = false;
     }
     public CharacterController m_charactercontroller
     {
         get
         {
             return this.transform.root.GetComponent<CharacterController>();
         }
     }
 }