php if/else shorthand fail

Normal expression works fine, shorthand doesn't. Where am I wrong here?

if (isset($var)) $value = $var;
elseif ($str !== 'string') $value = $str;
else $value = null;
// works just fine

$value = (isset($var)) ? $var : ($str !== 'string') ? $str : null;
// only returns $value = $str

Thanks