Creating a unique constraint on a PostGIS 'raster' type column

I am using the following command to add constraints to one of the raster image in PostGIS-2.1.3 (PostgreSQL-9.1.14). ALTER TABLE schema1.table1 ADD CONSTRAINT enforce_scalex_rast unique (rast); But getting the following errors: ERROR: data type raster has no default operator class for access method "btree" HINT: You must specify an operator class for the index or define a default operator class for the data type. Kindly someone help me to fix this error up. I have no basic idea about the operator classes. Thx. Zia.