casting LPARAM to a structure

We have a small problem with a project being converted from 32 bit to 64

The code fragment below is intended to fill a structure from a memory location passed by a message as an LPARAM

When we try in 64bit we get an invalid variable with the message "unable to read memory"

What are we doing wrong? How do we get it to work?

Any help gratefully received

Thank you

LONG_PTR CNexView::OnFindLink(WPARAM wParam, LPARAM data )

{
 CFindLinkData *ld = (CFindLinkData*)data;

}