I am trying to use the "findControl" but not having any success. I can find a control using the Master., but not on a content page.. What am I doing wrong?

I am trying to find a control on a content page from the content page codebehind. I can find controls on the master page adding a Master. to the find control, but for some reason can't find them on the content page.
Here is my code.
al.Add("hf0") al.Add("hf1") al.Add("hf2") al.Add("hf3") al.Add("hf4") For i = 0 To al.Count - 1 Dim hfTemp As HiddenField = CType(FindControl(al(i)), HiddenField) Dim hfTempValue as string = hfTemp.value Next I get an error Object reference not set to an instance of an object.