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 (else what 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