[plt-scheme] define-datatype error

Hi:

   I typed the following statement in DrScheme 101:

(define-datatype expression expression?
  (lit-exp    (datum number?))
  (var-exp    (id symbol?))
  (lambda-exp (id symbol?)
              (body expression?))
  (app-exp    (rator expression?)
              (rand expression?)))

I got the following error:  reference to undefined identifier: define-datatype
How can I fix this error? Thanks.