Laravel hasMany relation returns ' Not unique table/alias' error

Context

In Laravel/Lumen 5.5 I have a many-to-many relation defined in my model class SchemaTypes, using a pivot tabel schema_parent_type:

public function parents()
{
    return $this->belongsToMany('SchemaParentType', 'schema_parent_type', 'type_id', 'parent_id' );

}

I can attach a new parent without problems using this code:

$type = SchemaTypes::where('name', $parentName)->first();
if ( $type)
{
      $type->parents()->attach($parent->id);
}

When I check the table schema_parent_type after attaching a new 'parent' it looks perfect.

Problem

The problem arises when I want to query for all parents given a specific object. When I use this code:

$type = SchemaTypes::where('name', $parentName)->first();
$parents = $type->parents()->get();

this results in the following error:

SQLSTATE[42000]: Syntax error or access violation: 
1066 Not unique table/alias: 'schema_parent_type' 
(SQL: select `schema_parent_type`.*, `schema_parent_type`.`type_id` as `pivot_type_id`, `schema_parent_type`.`parent_id` as `pivot_parent_id` from `schema_parent_type` inner join `schema_parent_type` on `schema_parent_type`.`id` = `schema_parent_type`.`parent_id` where `schema_parent_type`.`type_id` = 110 and `schema_parent_type`.`deleted_at` is null)

Question

What is the reason that calling $type->parents()->get() produces this error? How can I solve this?

Any comments or answers are welcome.