Derma Menu - Change class to default class(citizen) when this button is pressed

[Slick]

Not sure what I'm doing. Basically, the 2nd button should change the players class to the default class which is citizen if pressed. It's not giving me the output I want. This is for darkrp.
Code:
[lua]
function MyCoolMenu()
local Frame = vgui.Create( "DFrame" )
Frame:SetTitle( "Make a person a cop" )
Frame:SetSize( 300, 500 )
Frame:Center()
Frame:MakePopup()

local Button = vgui.Create( "DButton", Frame )
Button:SetText( "test1" )
Button:SetTextColor( Color( 255, 255, 255 ) )
Button:SetPos( 100, 50 )
Button:SetSize( 100, 30 )
Button.Paint = function( self, w, h )
draw.RoundedBox( 0, 0, 0, w, h, Color( 41, 128, 185, 250 ) ) -- Draw a blue button
end
Button.DoClick = function( ply, team )
ply:changeTeam(GAMEMODE.DefaultTeam, true)
end

local Button = vgui.Create( "DButton", Frame )
Button:SetText( "test2" )
Button:SetTextColor( Color( 255, 255, 255 ) )
Button:SetPos( 100, 100 )
Button:SetSize( 100, 30 )
Button.Paint = function( self, w, h )
draw.RoundedBox( 0, 0, 0, w, h, Color( 41, 128, 185, 250 ) ) -- Draw a blue button
end
Button.DoClick = function(ply)

ply:changeTeam(GAMEMODE.DefaultTeam, true)
end
end
concommand.Add( "open_coolmenu", MyCoolMenu )
[/lua]

Console Error:
[lua]
[ERROR] gamemodes/darkrp/gamemode/cl_init.lua:56: attempt to call method 'changeTeam' (a nil value)
1. DoClick - gamemodes/darkrp/gamemode/cl_init.lua:56
2. unknown - lua/vgui/dlabel.lua:218

[/lua]

[NiandraLades]

ChangeTeam is probably a serverside only function, whereas derma menus are clientside - look into using the net library