public function setPrompt($prompt)
	{
		if (empty($prompt)) {
			return $this;
		}

		if (isset($this->items)) {
			if (in_array(NULL, array_keys($this->items))) {
				throw new InvalidArgumentException(
					"There is an item whose value == null (non-strict comparison)." .
					"Setting prompt would interfere with this value.");
			}
		}
		else {
			throw new NotSupportedException('This must be a ChoiceControl');
		}

		$this->prompt = $prompt;

		return $this;
	}