diff --git a/src/Speller.php b/src/Speller.php
index 7c2e84e..f06c2c2 100644
--- a/src/Speller.php
+++ b/src/Speller.php
@@ -241,7 +241,7 @@ protected abstract function getCurrencyName($type, $number, $currency);
 	
 	private static function assertNumber($number)
 	{
-		if (!is_numeric($amount))
+		if (!is_numeric($number))
 		{
 			throw new InvalidArgumentException('Invalid number specified.');
 		}