It's actually caused by the safety lug being cut too much and not properly contacting the sear.
If you take the grip safety off, reinstall the safety and repeat the test, you will see the sear move up which allows the hammer to fall when you disengage the safety.
There is an article on this and here is the link.
http://www.brazoscustom.com/magart/0601.htm
There are two ways to fix the problem.
You can peen the safety lug a little at a time to expand it and try to get the correct contact, or buy a new safety and have it fitted.
If the gun is new and custom build, I would contact the builder and have them replace the safety!