Primary key on NUMERIC type

Isn't it possible to put a PRIMARY KEY on a NUMERIC type?
Here is what I do:
   CREATE TABLE MNT_TEXTOS(       TEX_CODIGO              NUMERIC(5) NOT NULL,       CONSTRAINT C_TEX PRIMARY KEY
(TEX_CODIGO));

And I get this error message:
   ERROR:  Can't find a default operator class for type 1700.

What could be the problem?
Thanks in advance,
           Esteban Chiner

------------------------------------------------------------Esteban Chiner Sanz        mailto (work):
echiner@tissat.esTISSATAv.Aragon, 30, 5ª planta  Phone: 96 393 9950Valencia (SPAIN)