Setting window property causing X error

[Charlie]

I am writing a clipboard handler for X and am confused on what the
ICCCM section 2.6.2 means when it says:

"If the owner fails to convert the target named by an atom in the
MULTIPLE property, it should replace that atom in the property with
None."

I assumed None meant for the property type so I wrote this code:

XChangeProperty (g_display, info.xid, info.property,
None, sizeof (Atom) << 3, PropModeReplace,
(unsigned char*) info.data, 0);

But is causes a BadAtom error. If I substitute None with a valid
atom the error goes away. I tried using a 1 instead of a 0 for
nelements but still get the error.

Can someone please explain what the ICCCM is wanting me to do?

Thanks in advance,
Charlie