GetComputerName / vb.net

Hi All
i have problem when getcomputername

my code

Dim buffer As String
buffer = Space$(512)
Dim size As Long = buffer.Length
If GetComputerName(buffer, size) Then
x_computer_name = Mid(buffer, 1, size)
End If

error : Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

can you help me.
thank's