Compile error

I'm getting the following error after upgrading to a new computer:

"The code in this project must be updated for use on 64-bit systems.  Please review and update Declare statements and then mark them with PtrSafe attribute."

This is the code that is failing:

Private Declare Function apiGetUserName Lib "advapi32.dll" Alias _
    "GetUserNameA" (ByVal lpBuffer As String, nSize As Long) As Long

How do I fix this?