PInvoke - Issue while calling DJVU function from C# Code - Attempted to read or write protected memory

Hi,

I know there are many questions in this subject but none of them help to resolve the issue I am currently facing.

Below is the signature of C Function from DJVULibre added in .NET code

[DllImport("C:\\Program Files\\DJVULIBRE\\LIBDJVULIBRE.dll", CharSet=CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
private unsafe static extern int ddjvu_page_render(IntPtr page, ddjvu_render_mode_t mode, IntPtr pagerect,
          IntPtr renderrect,
          IntPtr pixelformat,
          ulong rowsize,
          [Out][MarshalAs(UnmanagedType.LPArray)]byte[] imagebuffer);
Below is how I am calling this function in the c# code
byte* buffer = (byte *)Memory.Alloc(nSize);
            try
            {
                IntPtr ptr1 = (IntPtr)Memory.Alloc(Marshal.SizeOf(prect));
                Marshal.StructureToPtr(prect, ptr1, false);

                IntPtr ptr2 = (IntPtr)Memory.Alloc(Marshal.SizeOf(rrect));
                Marshal.StructureToPtr(rrect, ptr2, false);

                byte[] array = new byte[nSize];
                fixed (byte* p = array) Memory.Copy(buffer, p, nSize);
                ddjvu_page_render(page, ddjvu_render_mode_t.DDJVU_RENDER_MASKONLY, ptr1, ptr2, fmt, (ulong)stride, array);
            }
            finally
            {
                Memory.Free(buffer);
            }
call to ddjvu_page_render in above code is throwing "Attempted to read or write protected memory. This is often an indication that other memory is corrupt."
Prior to this post I must have tried all the option could find in various blogs.
Appreciate any help, is almost a day I am clueless, your timely help could save my job