How to add carryover to a 2x2 crossover model?

[MaryA_Marion]

I am learning to do 2x2 crossover designs. carryover is coded with 2 levels. When period=1, carryover=X and when period=2, carryover=1.

When I add carryover as a factor I am getting 0 df and SS's? Why? Please see code enclosed. 

title 'table 3.6.3, page 75 Chou and Liu -Design and Analysis of Bioavailability and Bioequivalence Studies, Third edition';
;

proc glm data=mydata;
class group subject tmt period carryover;
model y=group subject (group) period tmt carryover /ss1 ss2 ss3 ss4 solution p;
test h=group e=subject(group) / htype=3 etype=3;
run;