Tree go previous node and go next node ?

[ButtonMaker]

there is a problem with my code but i couldnt find it !!! i have 2 buttons "previous" and "next" for tree object;

previous button code :

nCount = Tree.GetChildCount("Tree1", "0");
if (nCount > 0) then
Tree.SetSelectedNode("Tree1", tostring(nInd));
    MediaPlayer.Load("media1", Tree.GetNode("Tree1", nInd).Data);
    if (nInd == nCount) then
        nInd = 1;
    else
		nInd = nInd - 1;
    end
end

next button code :

nCount = Tree.GetChildCount("Tree1", "0");
if (nCount > 0) then
Tree.SetSelectedNode("Tree1", tostring(nInd));
    MediaPlayer.Load("media1", Tree.GetNode("Tree1", nInd).Data);
    if (nInd == nCount) then
        nInd = 1;
    else
		nInd = nInd + 1;
    end
end

when i click next button it works ok and jumps the next node but once if i click the previous button it jumps to first node not the one before selected and then next button gives the same error !!!

[Kick]

* On Click the button "Previous":

local strSelectedNode = Tree.GetSelectedNode("Tree1");
if (strSelectedNode ~= "") then
    if (strSelectedNode == "1") then
        nInd = Tree.GetChildCount("Tree1", "0");
    else
        nInd = tonumber(strSelectedNode) - 1;
    end
    Tree.SetSelectedNode("Tree1", tostring(nInd));
    --MediaPlayer.Load("media1", Tree.GetNode("Tree1", tostring(nInd)).Data);
else
    Tree.SetSelectedNode("Tree1", "1");
end

* On Click the button "Next":

local strSelectedNode = Tree.GetSelectedNode("Tree1");
if (strSelectedNode ~= "") then
    if (tonumber(strSelectedNode) == Tree.GetChildCount("Tree1", "0")) then
        nInd = 1;
    else
        nInd = tonumber(strSelectedNode) + 1;
    end
    Tree.SetSelectedNode("Tree1", tostring(nInd));
    --MediaPlayer.Load("media1", Tree.GetNode("Tree1", tostring(nInd)).Data);
else
    Tree.SetSelectedNode("Tree1", "1");
end