Floats and thus the function floatval() used to test filtered value in Zend_Validate_Float::isValid() is locale aware and return value with localized decimal_point. Thus the test will always fail when a locale is set which uses a decimal point other than ".". Tested with PHP 5.2.6.2. When... setlocale(LC_ALL, 'sv_SE.UTF-8'); $value = 10.5; ...this will always fail as it results in a comparison between "10,5" and "10.5". if (strval(floatval($valueFiltered)) != $valueFiltered) { $this->_error(); return false; } Proposed solution list($num, $dec) = explode('.', $valueFiltered); if (strval(intval($num)) != $num || strval(intval($dec)) != $dec) { $this->_error(); return false; } There are also no unit test Zend_Validate_Float with a locale other than default.