According to the magazine, Robert gave his on-and-off screen girlfriend an "ultimatum" to prove she is committed to him.
The magazine reports: "But don’t expect a big announcement any time soon!”
They insisted the couple are planning "to keep things under wraps for a while."
“The whole engagement was a very spur-of-the moment thing. Rob didn’t even have time to buy Kristen a ring," they reported.
We are so not so convinced.