creating index works OK on one db, not on other

I have a problem with creating a specific type of index in postgres.
Within the same system, it is created OK on one database but not on another.

CREATE INDEX key2img_fr_rdtree_idx on image_fr using  gist ( key2img
gist__intbig_ops) with ( islossy );

works on one database but on the other one, I get this:
ERROR:  DefineIndex: opclass "gist__intbig_ops" not found


any ideas on what I need to do to get this working?

k.


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