i cannot call null data row in sqlite

Why I cannot call null data row when I use IS NULL in sqlite

select roomkeyID from roomkey where dateOfReturn is null;

CREATE TABLE ROOMKEY ( 
    roomkeyID VARCHAR(10) not null primary key,
    roomID VARCHAR(10) not null, 
    keyID VARCHAR(10) not null,
    dateOfIssue DATETIME,
    dateOfReturn DATETIME,
    FOREIGN KEY (roomID) REFERENCES ROOM(roomID),
    FOREIGN KEY (keyID ) REFERENCES KEY(keyID )
)

"RK000001","R106","K0003","12/17/2015"," "
"RK000090","R101","K0002","12/12/2015"," "