C# p/invoke Secur32.dll problems

Question

I'm trying to wrap the Secur32.dll's EnumerateSecurityPackages function which is declared below:

SECURITY_STATUS SEC_Entry EnumerateSecurityPackages(
  __in  PULONG pcPackages,
  __in  PSecPkgInfo *ppPackageInfo
);

I have the following C# code, but I get an AccessViolationException when I try to run it. In the debugger the pcPackages variable does get set correctly, but I think I'm doing something wrong with the array of SecPkgInfos.

[StructLayout(LayoutKind.Sequential)]
public struct SecPkgInfo
{
    public ulong fCapabilities;
    public ushort wVersion;
    public ushort wRPCID;
    public ulong cbMaxToken;
    public string Name;
    public string Comment;
}

[DllImport("Secur32.dll")]
public extern static int EnumerateSecurityPackages(
    ref ulong pcPackages,
    ref SecPkgInfo[] ppPackageInfo
);

///Calling code
ulong count = 0;
SecPkgInfo[] buffer = new SecPkgInfo[256];
EnumerateSecurityPackages(ref count, ref buffer);

Any ideas what I'm doing wrong?