assigning to NULL?

I'm trying to make postGIS work with pg7.3devel. But a problem is occuring that did not appear in pg7.2. When I
execute:

ALTER TABLE geotest ADD CHECK ( geometrytype(geopoint)='POINT'
OR NULL=geopoint);

I get: "ERROR: copyObject: don't know how to copy node type 506"

But when I execute:

ALTER TABLE geotest ADD CHECK ( geometrytype(geopoint)='POINT');

It works fine, which, due to the error message it seems that it is trying to assign rather to NULL, rather than compare
(elsewhat object needs to be copied in "NULL=geopoint"?). Is this a bug, a change in NULL, or a change in user defined
datatypes?
Thanks;
Eric