Shut off doesn't fit

[pigboy13]

I'm replacing a 45 year old shut off for my mother, under the kitchen sink. the water line that comes out of the wall has what appears to be a 1/2 inch chrome nipple with threads. I cannot see how it is attached inside the wall but i know i connects to 1/2 copper. My issue is the New Shut off, quarter turn, 1/2-in FIP x 3/8-in OD compression, slides right over the 1/2 nipple and does not engage the threads at all. I tested a 3/8 in nut on the nipple to make sure i wasn't losing my mind and it didn't come close to fitting, way to small. So what kind of shut off do i need or what kind of "adapters" do in need to make this work.

 

[Reach4]

A nominal 1/2 inch nipple is about 0.84 inches OD at widest.

A nominal 3/8 inch (NPT or MIP or MPT) nipple is about 0.675 inches OD.

I suspect that your nipple is 3/8.