SQLite SELECT not returning anything

This works to return a name:

SELECT Fname, Lname FROM EMPLOYEE WHERE dno = 4 AND Ssn = 999887777;

returns the employee's name.

But this returns nothing:

SELECT Fname, Lname FROM EMPLOYEE WHERE dno = 4 AND Ssn = (SELECT essn FROM WORKS_ON);

I want to get the names of all employees in Dept. No. 4 whose employee SSNs are found in the WORKS_ON table.

'essn' and 'Ssn' are both 9-character text fields that work in other contexts.