private function normalizeExpandKeys($expandKeys)
	{
		// clean up expand keys
		$expandKeys = array_map('trim', explode(',', $expandKeys));
		$normalized = [];

		foreach ($this->acceptedKeys as $type => $acceptedKeys)
		{
			foreach ($expandKeys as $expandKey)
			{
				if (in_array($expandKey, $acceptedKeys))
				{
					$normalized[$expandKey] = $type;
				}
			}
		}

		return $normalized;
	}