failure with RANGE-MIN-CARDINALITY

Hello,
My assignment to RANGE-MIN-CARDINALITY is effectless:
|= (defconcept Person)

|c|PERSON

|= (defrelation rel ((?p1 Person)(?p2 Person)))

|r|REL

|= (retrieve all ?x (RANGE-MIN-CARDINALITY rel Person ?x))

Processing check-types agenda...
There is 1 solution:
  #1: ?X=0

|= (assert (RANGE-MIN-CARDINALITY rel Person 1))

|P|(= (RANGE-MIN-CARDINALITY REL PERSON) 1)

|= (retrieve all ?x (RANGE-MIN-CARDINALITY rel Person ?x))

There is 1 solution:
  #1: ?X=0

But RANGE-MAX-CARDINALITY DID work as I've expected. Anyone has ideas
about this? Thanks a lot.

--
Pan Xingzhi