Trying to combine multiple ManyToMany Relationships

gistfile1.php

  public function sponsor()
	{
		$sponsor = $this->belongsToMany('Group')->first();

		if(!$sponsor) {
			return $this->belongsToMany('User');
		}
		
		return $this->belongsToMany('Group'); 
		
	}
	
	public function userSponsor()
	{
		return $this->belongsToMany('User');
	}
	
	public function groupSponsor()
	{
		return $this->belongsToMany('Group');
	}

The interface limits the sponsor to either a User or a Group.

Simply trying to call sponsor() should return either the User sponsor or the Group sponsor. Currently it won't return a User sponsor.