Adding a FK for column that have NULL values gives a constra

Progress version: OpenEdge Progress 10.1B02

Adding a foreign key for column that have NULL values gives a constraint violation.

From other side its possible to set NULL value for already existing foreign key.

Example:

create table pub."Test1"
(
"Field1" number not null,
"Field2" varchar(100),
PRIMARY KEY ( "Field1" )
)

insert into pub."Test1" values (1, 'Value 1')
insert into pub."Test1" values (2, 'Value 2')
insert into pub."Test1" values (3, 'Value 3')
insert into pub."Test1" values (4, 'Value 4')

select * from pub."Test1"

create table pub."Test2"
(
"Field1" number not null,
"Field2" number,
"Field3" varchar(100),
PRIMARY KEY ( "Field1" ),
CONSTRAINT "Fk_Test2_Test1_Field2" FOREIGN KEY ( "Field2" ) REFERENCES pub."Test1" ( "Field1" )
)

insert into pub."Test2" values (1,1, 'Some Value 1')
insert into pub."Test2" values (2,1, 'Some Value 2')
insert into pub."Test2" values (3,2, 'Some Value 3')
insert into pub."Test2" values (4,2, 'Some Value 4')
insert into pub."Test2" values (5,3, 'Some Value 5')
insert into pub."Test2" values (6,3, 'Some Value 6')
insert into pub."Test2" values (7,null, 'Some Value 7')
insert into pub."Test2" values (8,null, 'Some Value 8')

select * from pub."Test2"

alter table pub."Test2"
drop CONSTRAINT "Fk_Test2_Test1_Field2"

alter table pub."Test2"
add CONSTRAINT "Fk_Test2_Test1_Field2"
FOREIGN KEY ( "Field2" ) REFERENCES pub."Test1" ( "Field1" )

The last sql statement gives following error: Constraint violation (7597)

How can I solve this problem?