"EMPID: invalid identifier"?

[nick-cd]

This CREATE TABLE commands as well as a couple of others throw this error

CREATE TABLE a2professors (

empID NUMBER(5, 0),
deptCode NUMBER(3, 0),
isActive NUMBER(1, 0),
    CONSTRAINT a2professors_isActive_ck CHECK(isActive IN(1, 0)),
    CONSTRAINT a2professors_empID_pk PRIMARY KEY(empID),
    CONSTRAINT a2professors_empID_fk FOREIGN KEY(empID)
        REFERENCES employees(empID),
    CONSTRAINT a2professors_deptCode_fk FOREIGN KEY(deptCode)
        REFERENCES departments(deptCode)

);