Math question sometimes fails.

Just installed captcha on the contact form of one of my 4.7.5 sites.

I test the form as an anon user. Sometimes when I enter the correct answer to the math question captcha returns an error: "The answer you entered to the math problem is incorrect.".

But I am SURE 7 + 7 IS 14. :-)

Often this is the case the first time I submit the form then on subsequent attempts the correct answer is accepted. But sometimes I get the error message on subsequent attempts and sometimes the correct answer is accepted first time.

I note that RobRoy is having a similar problem, reported at http://drupal.org/node/106235 #20