Problem with TabControl

[matatu]

i create a button in TabControl1 with a name Button1

when i use

Button1.BackColor = System.Drawing.Color.MintCream

it is working fine but when i use (any of the following)

dim i as integer=1

Controls(String.Concat("Button", i)).BackColor = System.Drawing.Color.MintCream

(or)

Controls(String.Concat("Button"& i)).BackColor = System.Drawing.Color.MintCream

it gives "An unhandled exception of type 'System.NullReferenceException' occurred in xxxx.exe" error with Additional information: Object reference not set to an instance of an object

what is the problem i didnt understand :S
(i am using vb 2005)