error en math-comment-spam-protection.classes.php

[Ascariz]

Me esta dando este error y no se por que puede ser?

Warning: array_rand() [function.array-rand]: Second argument has to be between 1 and the number of elements in the array in httpdocs\blog\wp-content\plugins\math-comment-spam-protection\math-comment-spam-protection.classes.php on line 71

Código:

function GenerateValues() {

		// Get numbers in array
		$num_array = $this->auxNoToArray($this->opt['input_numbers']);
		// Get random keys
		$rand_keys = array_rand($num_array, 2); ERROR aki

		// Operands for displaying...
		$this->info['operand1'] = $num_array[$rand_keys[0]];
		$this->info['operand2'] = $num_array[$rand_keys[1]];