diff --git a/phpBB/includes/captcha/captcha_gd.php b/phpBB/includes/captcha/captcha_gd.php
index 7a3f4f46ab..6d859a4ecc 100644
--- a/phpBB/includes/captcha/captcha_gd.php
+++ b/phpBB/includes/captcha/captcha_gd.php
@@ -77,7 +77,7 @@ class captcha
 		{
 			$denom = ($code_len - $i);
 			$denom = max(1.3, $denom);
-			$offset[$i] = mt_rand(0, (int) round((1.5 * $width_avail) / $denom));
+			$offset[$i] = phpbb_mt_rand(0, (int) round((1.5 * $width_avail) / $denom));
 			$width_avail -= $offset[$i];
 		}